Add two new hooks to allocator_extension.h

Add two hooks to allocator_extension.h:
1) SetNumericProperty() to set the value associated with a given property;
2) GetHeapSample() to retrieve a sample of live objects and the stack
   traces that allocated those objects.

BUG=b:121160411

Change-Id: I3cdc8edd906c8d4ddec2f1394b2862e94a86451d
Reviewed-on: https://chromium-review.googlesource.com/c/1446796
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630944}
2 files changed