Oilpan: GC profiling.

This CL adds GC memory profiling using the about:tracing infrastructure. The
profiling can be time consuming, so the GC_PROFILE_* macros are used to
selectively enable various profiling features for the time being.

The GC_TRACING macro has been renamed to GC_PROFILE_MARKING to avoid confusion with about:tracing. The tracing event group has been renamed from blink to blink_gc to allow collection of GC related events independent of other blink events.

BUG=

Review URL: https://codereview.chromium.org/383743002

git-svn-id: svn://svn.chromium.org/blink/trunk@180297 bbb929c8-8fbe-4397-9dbb-9b2b20218538