This patch is a follow-up patch to enable gc statistics to use
TracingCategoryObserver.

Previously we need to pass --track_gc_object_stats to v8 if we want to enable
gc statistics in tracing. In this patch, we introducce an integer flag
FLAG_gc_stats, and FLAG_track_gc_object_stats and FLAG_trace_gc_object_stats
will set it to 0x01, tracing will set it to 0x10 when we start tracing and
reset the bit when we stop tracing.

BUG=v8:5590

Review-Url: https://codereview.chromium.org/2459903003
Cr-Commit-Position: refs/heads/master@{#40777}