Currently we relay all allow{Database,FileSystem,IndexedDB} requests for 
workers from worker thread to renderer thread, but this introduces 
a lot of complexity and leak/u-a-f problems. This change tries to 
introduce WebWorkerPermissionClientProxy interface, which gets created 
on the renderer thread, passed on to the worker thread and then issues 
sync IPC (on the worker thread) in the embedder code.

This will allow us to remove a lot of code (in the next iteration):
- To get rid of complex (and insecure) WorkerAllowMainThreadBridge code
- To remove more WorkerRunLoop.runInMode usage
- To simplify worker client inheritance chain

Chromium-side change: https://codereview.chromium.org/46583005/

BUG=none

Review URL: https://codereview.chromium.org/50773002

git-svn-id: svn://svn.chromium.org/blink/trunk@161017 bbb929c8-8fbe-4397-9dbb-9b2b20218538