This moves another root scanning task out of the GC coordinator and
parallelizes it on the GC workers.

This has negligible effect on the go1 benchmarks and the garbage
benchmark.

name              old time/op  new time/op  delta
XBenchGarbage-12  5.24ms ± 1%  5.26ms ± 1%  +0.30%  (p=0.007 n=18+17)

name                      old time/op    new time/op    delta
BinaryTree17-12              3.20s ± 5%     3.21s ± 5%    ~     (p=0.264 n=20+18)
Fannkuch11-12                2.46s ± 1%     2.54s ± 2%  +3.09%  (p=0.000 n=18+20)
FmtFprintfEmpty-12          49.9ns ± 4%    50.0ns ± 5%    ~     (p=0.356 n=20+20)
FmtFprintfString-12          170ns ± 1%     170ns ± 2%    ~     (p=0.815 n=19+20)
FmtFprintfInt-12             160ns ± 1%     159ns ± 1%  -0.63%  (p=0.003 n=18+19)
FmtFprintfIntInt-12          270ns ± 1%     267ns ± 1%  -1.00%  (p=0.000 n=19+18)
FmtFprintfPrefixedInt-12     238ns ± 1%     232ns ± 1%  -2.28%  (p=0.000 n=19+19)
FmtFprintfFloat-12           310ns ± 2%     313ns ± 2%  +0.93%  (p=0.000 n=19+19)
FmtManyArgs-12              1.06µs ± 1%    1.04µs ± 1%  -1.93%  (p=0.000 n=20+19)
GobDecode-12                8.63ms ± 1%    8.70ms ± 1%  +0.81%  (p=0.001 n=20+19)
GobEncode-12                6.52ms ± 1%    6.56ms ± 1%  +0.66%  (p=0.000 n=20+19)
Gzip-12                      318ms ± 1%     319ms ± 1%    ~     (p=0.405 n=17+18)
Gunzip-12                   42.1ms ± 2%    42.0ms ± 1%    ~     (p=0.771 n=20+19)
HTTPClientServer-12         62.6µs ± 1%    62.9µs ± 1%  +0.41%  (p=0.038 n=20+20)
JSONEncode-12               16.9ms ± 1%    16.9ms ± 1%    ~     (p=0.077 n=18+20)
JSONDecode-12               60.7ms ± 1%    62.3ms ± 1%  +2.73%  (p=0.000 n=20+20)
Mandelbrot200-12            3.86ms ± 1%    3.85ms ± 1%    ~     (p=0.084 n=19+20)
GoParse-12                  3.75ms ± 2%    3.73ms ± 1%    ~     (p=0.107 n=20+19)
RegexpMatchEasy0_32-12       100ns ± 2%     101ns ± 2%  +0.97%  (p=0.001 n=20+19)
RegexpMatchEasy0_1K-12       342ns ± 2%     332ns ± 2%  -2.86%  (p=0.000 n=19+19)
RegexpMatchEasy1_32-12      83.2ns ± 2%    82.8ns ± 2%    ~     (p=0.108 n=19+20)
RegexpMatchEasy1_1K-12       495ns ± 2%     490ns ± 2%  -1.04%  (p=0.000 n=18+19)
RegexpMatchMedium_32-12      130ns ± 2%     131ns ± 2%    ~     (p=0.291 n=20+20)
RegexpMatchMedium_1K-12     39.3µs ± 1%    39.9µs ± 1%  +1.54%  (p=0.000 n=18+20)
RegexpMatchHard_32-12       2.02µs ± 1%    2.05µs ± 2%  +1.19%  (p=0.000 n=19+19)
RegexpMatchHard_1K-12       60.9µs ± 1%    61.5µs ± 1%  +0.99%  (p=0.000 n=18+18)
Revcomp-12                   535ms ± 1%     531ms ± 1%  -0.82%  (p=0.000 n=17+17)
Template-12                 73.0ms ± 1%    74.1ms ± 1%  +1.47%  (p=0.000 n=20+20)
TimeParse-12                 356ns ± 2%     348ns ± 1%  -2.30%  (p=0.000 n=20+20)
TimeFormat-12                347ns ± 1%     353ns ± 1%  +1.68%  (p=0.000 n=19+20)
[Geo mean]                  62.3µs         62.4µs       +0.12%

name                      old speed      new speed      delta
GobDecode-12              88.9MB/s ± 1%  88.2MB/s ± 1%  -0.81%  (p=0.001 n=20+19)
GobEncode-12               118MB/s ± 1%   117MB/s ± 1%  -0.66%  (p=0.000 n=20+19)
Gzip-12                   60.9MB/s ± 1%  60.8MB/s ± 1%    ~     (p=0.409 n=17+18)
Gunzip-12                  461MB/s ± 2%   462MB/s ± 1%    ~     (p=0.765 n=20+19)
JSONEncode-12              115MB/s ± 1%   115MB/s ± 1%    ~     (p=0.078 n=18+20)
JSONDecode-12             32.0MB/s ± 1%  31.1MB/s ± 1%  -2.65%  (p=0.000 n=20+20)
GoParse-12                15.5MB/s ± 2%  15.5MB/s ± 1%    ~     (p=0.111 n=20+19)
RegexpMatchEasy0_32-12     318MB/s ± 2%   314MB/s ± 2%  -1.27%  (p=0.000 n=20+19)
RegexpMatchEasy0_1K-12    2.99GB/s ± 1%  3.08GB/s ± 2%  +2.94%  (p=0.000 n=19+19)
RegexpMatchEasy1_32-12     385MB/s ± 2%   386MB/s ± 2%    ~     (p=0.105 n=19+20)
RegexpMatchEasy1_1K-12    2.07GB/s ± 1%  2.09GB/s ± 2%  +1.06%  (p=0.000 n=18+19)
RegexpMatchMedium_32-12   7.64MB/s ± 2%  7.61MB/s ± 1%    ~     (p=0.179 n=20+20)
RegexpMatchMedium_1K-12   26.1MB/s ± 1%  25.7MB/s ± 1%  -1.52%  (p=0.000 n=18+20)
RegexpMatchHard_32-12     15.8MB/s ± 1%  15.6MB/s ± 2%  -1.18%  (p=0.000 n=19+19)
RegexpMatchHard_1K-12     16.8MB/s ± 2%  16.6MB/s ± 1%  -0.90%  (p=0.000 n=19+18)
Revcomp-12                 475MB/s ± 1%   479MB/s ± 1%  +0.83%  (p=0.000 n=17+17)
Template-12               26.6MB/s ± 1%  26.2MB/s ± 1%  -1.45%  (p=0.000 n=20+20)
[Geo mean]                99.0MB/s       98.7MB/s       -0.32%

Change-Id: I6ea44d7a59aaa6851c64695277ab65645ff9d32e
Reviewed-on: https://go-review.googlesource.com/16070Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>