Currently this leads to a significant skew towards 'etext' entry,
since all idle threads are profiled every tick.
Before:
Total: 66608 samples
   63188  94.9%  94.9%    63188  94.9% etext
     278   0.4%  95.3%      278   0.4% sweepspan
     216   0.3%  95.6%      448   0.7% runtime.mallocgc
     122   0.2%  95.8%      122   0.2% scanblock
     113   0.2%  96.0%      113   0.2% net/textproto.canonicalMIMEHeaderKey
After:
Total: 8008 samples
    3949  49.3%  49.3%     3949  49.3% etext
     231   2.9%  52.2%      231   2.9% scanblock
     211   2.6%  54.8%      211   2.6% runtime.cas64
     182   2.3%  57.1%      408   5.1% runtime.mallocgc
     178   2.2%  59.3%      178   2.2% runtime.atomicload64

LGTM=alex.brainman
R=golang-codereviews, alex.brainman
CC=golang-codereviews
https://golang.org/cl/61250043