Implement WebThreadSupportingGC, which wraps a WebThread attached to Oilpan's GC

In order to attach/detach a WebThread to/from Oilpan's GC,
we need a bunch of setups about PendingGCRunner, MessageLoopInterruptor etc.
To avoid duplicating the setup code in all WebThread customers,
this CL adds a thin wrapper class (WebThreadSupportingGC) for a WebThread
attached to Oilpan's GC.

BUG=340522

Review URL: https://codereview.chromium.org/469683002

git-svn-id: svn://svn.chromium.org/blink/trunk@182385 bbb929c8-8fbe-4397-9dbb-9b2b20218538