Automatically take heap snaphot on HeapProfiler.stopTrackingHeapObjects command

HeapSnapshot is taking right before heap objects tracking is switched off. We anyways take the snapsot every time when stopping tracking and it is important to make this atomically so that no additional allocations slipped through.

Also stopTrackingHeapObjects will send heap stats update for the last (unfinished) time interval.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/98423004

git-svn-id: svn://svn.chromium.org/blink/trunk@162984 bbb929c8-8fbe-4397-9dbb-9b2b20218538