Add IPC stubs between browser and ServiceWorker's worker context in the child process

- Add ServiceWorkerHostMsg_WorkerStarted and ServiceWorkerHostMsg_WorkerStopped
  that are sent from embedded worker to the browser process
- Add ServiceWorkerContextMsg_FetchEvent for messages from embedded worker
  to the browser process

FetchEvent/FetchRequest code is a bit rough as it's meant to be
a placeholder.  I can remove the part from this CL if it doesn't
look reay yet.

The new code uses new IPC class (ServiceWorkerContextMsg) for dispatching
messages specifically sent to the embedded workers's worker context,
which may feel a bit noisy. (Alternative suggestions welcome)

BUG=313530
R=alecflett@chromium.org, jochen@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/118103002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243081 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243081