Adding api to get last gc object statistics for chrome://tracing.

Adding api to get last gc object statistics for chrome://tracing.

For dumping the number of objects and size of objects alive after the
last gc into chrome://tracing memory dumps, this CL adds new api to
get these number for each isolate.

Note: Chrome should be run with --js-flags='--track_gc_object_stats
--noincremental-marking' for object tracking.

BUG=476013
LOG=Y

Review URL: https://codereview.chromium.org/1113233002

Cr-Commit-Position: refs/heads/master@{#28474}