to return memory to the OS after completing potentially
large operations.

Update #21870

Sys went down to 3.7G

$ DEBUG_MEMORY_USAGE=1 go tool trace trace.out

2018/03/07 09:35:52 Parsing trace...
after parsing trace
 Alloc:	3385754360 Bytes
 Sys:	3662047864 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	3488907264 Bytes
 HeapInUse:	3426549760 Bytes
 HeapAlloc:	3385754360 Bytes
Enter to continue...
2018/03/07 09:36:09 Splitting trace...
after spliting trace
 Alloc:	3238309424 Bytes
 Sys:	3684410168 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	3488874496 Bytes
 HeapInUse:	3266461696 Bytes
 HeapAlloc:	3238309424 Bytes
Enter to continue...
2018/03/07 09:36:39 Opening browser. Trace viewer is listening on http://100.101.224.241:12345

after httpJsonTrace
 Alloc:	3000633872 Bytes
 Sys:	3693978424 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	3488743424 Bytes
 HeapInUse:	3030966272 Bytes
 HeapAlloc:	3000633872 Bytes
Enter to continue...

Change-Id: I56f64cae66c809cbfbad03fba7bd0d35494c1d04
Reviewed-on: https://go-review.googlesource.com/92376Reviewed-by: Peter Weinberger <pjw@google.com>