InstanceCounter/Oilpan GC tracing: Add clang support

This CL fixes two issues:
- Add clang support for InstanceCounter type resolver
- Add a mutex when writing to objectgraph map.
-- Oilpan GC marking phase is only run on single thread, however, weak processing are not.
-- Heap wtf collection implementations may mark their back store on weak processing phase.

BUG=378157
NOTRY=true

Review URL: https://codereview.chromium.org/304033004

git-svn-id: svn://svn.chromium.org/blink/trunk@175092 bbb929c8-8fbe-4397-9dbb-9b2b20218538