[NOT READY FOR REVIEW YET]workers: Some refactoring to allow creating new kind of worker threads

workers: Some refactoring to allow creating new kind of worker threads

Notable changes:

 * The WorkerThread is now responsible for creating the v8::Isolate for running
the script.
 * WorkerScriptController receives a v8::Isolate from the WorkerThread during
construction.
 * Allow sub-classing WebThreadSupportingGC. The default implementation
remains the same as before. But this will allow creating WebThreadSupportingGC
objects that share a single WebThread object (instead of each
WebThreadSupportingGC owning its own WebThread instance, which is the current
implementation).

BUG=none