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)

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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@31077 0039d316-1c4b-4281-b951-d872f2087c98