Oilpan: Remove adoptRefCountedGarbageCollected

adoptRefCountedGarbageCollected is no longer needed. When creating a RefCountedGarbageCollected object, we can just use 'new RefCountedGarbageCollectedObj()'. Even if a GC is triggered between when the object is created and when the object is adopted, there is no issue because our conservative scanning finds the object and traces it.

This CL also removes ThreadSafeRefCountedGarbageCollected because it is now unused (WebAudio was the only user of ThreadSafeRefCountedGarbageCollected, but we already shipped Oilpan for WebAudio and removed the need).

BUG=340522

Review URL: https://codereview.chromium.org/614373007

git-svn-id: svn://svn.chromium.org/blink/trunk@183582 bbb929c8-8fbe-4397-9dbb-9b2b20218538