Revert of [Tracing] Use TracingCategoryObserver in gc statistics (patchset #5 id:100001 of https://codereview.chromium.org/2459903003/ )

Reason for revert:
Speculative revert for blocking roll:
https://codereview.chromium.org/2473003006/

E.g.:
https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_rel_ng/builds/331898

Original issue's description:
> [Tracing] Use TracingCategoryObserver in gc statistics
>
> 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

TBR=fmeawad@chromium.org,mlippautz@chromium.org,lpy@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:5590

Review-Url: https://codereview.chromium.org/2477143002
Cr-Commit-Position: refs/heads/master@{#40785}