Initial child-process side plumbing for starting an embedded ServiceWorker context

Design assumptions (some assumptions can be tentative) for this CL:
* ServiceWorkerProvider is going to be embedded in all frame/document
* Browser process can tell which SWP is associated to which SW (if any),
  via SWP registration process that is not added yet
* A newly created embedded worker context will always create its own shadow page
 for its loading context
* A newly created embedded worker context stub will keep a ref to the embedding process

No browser-side implementation / blink plumbing yet.

This adds a new tiny dispatcher which listens messages
solely on the renderer thread.

BUG=313530

Review URL: https://codereview.chromium.org/54573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237873 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237873