We force runtime.GC before WriteHeapProfile with -test.heapprofile.
Make it possible to do the same with the HTTP interface.

Some servers only run a GC every few minutes.
On such servers, the heap profile will be a few minutes stale,
which may be too old to be useful.

Requested by private mail.

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews
https://golang.org/cl/161990043