Aggregated snapshots: don't report unreachable objects.
Full snapshots: restore forcing GC prior to taking a snapshot.

I played with the repro page provided for the bug and found that GC
must be performed prior to taking a snapshot even if we only report
reachable objects. GC allows weak handles to finalize.  Now heap
profiler produces aligned results for the repro page in both modes.

BUG=crbug/70434
TEST=none

Review URL: http://codereview.chromium.org/6410030

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6598 ce2b1a6d-e550-0410-aec6-3dcde31c8c00