Enabled when the tool runs with DEBUG_MEMORY_USAGE=1 env var.
After reporting the usage, it waits until user enters input
(helpful when checking top or other memory monitor)

Also adds net/http/pprof to export debug endpoints.

From the trace included in #21870

$ DEBUG_MEMORY_USAGE=1 go tool trace trace.out
2018/02/21 16:04:49 Parsing trace...
after parsing trace
 Alloc:	3385747848 Bytes
 Sys:	3661654648 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	3488907264 Bytes
 HeapInUse:	3426377728 Bytes
 HeapAlloc:	3385747848 Bytes
Enter to continue...
2018/02/21 16:05:09 Serializing trace...
after generating trace
 Alloc:	4908929616 Bytes
 Sys:	5319063640 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	5032411136 Bytes
 HeapInUse:	4982865920 Bytes
 HeapAlloc:	4908929616 Bytes
Enter to continue...
2018/02/21 16:05:18 Splitting trace...
after spliting trace
 Alloc:	4909026200 Bytes
 Sys:	5319063640 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	5032411136 Bytes
 HeapInUse:	4983046144 Bytes
 HeapAlloc:	4909026200 Bytes
Enter to continue...
2018/02/21 16:05:39 Opening browser. Trace viewer is listening on http://127.0.0.1:33661
after httpJsonTrace
 Alloc:	5288336048 Bytes
 Sys:	7790245896 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	7381123072 Bytes
 HeapInUse:	5324120064 Bytes
 HeapAlloc:	5288336048 Bytes
Enter to continue...

Change-Id: I88bb3cb1af3cb62e4643a8cbafd5823672b2e464
Reviewed-on: https://go-review.googlesource.com/92355Reviewed-by: Peter Weinberger <pjw@google.com>