Added beginnings of browser-process support for shared workers.

      Refactored WebWorkerClientProxy into two classes - WebWorkerDispatcher which dispatches incoming IPCs for the worker, and WebWorkerClientProxy, which handles outgoing API calls from WebWorkerImpl. This allows WebWorkerClientProxy to be reused by WebSharedWorkerDispatcher.

BUG=26233
TEST=none (will enable layout tests when basic functionality available)

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=31077

Review URL: http://codereview.chromium.org/351004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@31096 0039d316-1c4b-4281-b951-d872f2087c98