Add skeleton of the ThreadState abstraction for OilPan's heap.

It will be used to track per-thread state: garbage collection and sweeping requests, safepoint status, thread local heaps, stack limits, statistics etc.

Review URL: https://codereview.chromium.org/115113003

git-svn-id: svn://svn.chromium.org/blink/trunk@163889 bbb929c8-8fbe-4397-9dbb-9b2b20218538