Start of Chrome-side plumbing for ServiceWorker fetch

Now ServiceWorkerScriptContext::OnFetchEvent provides a dummy response
to the callback.

BUG=342301
TEST=content_browsertests:ServiceWorkerVersionBrowserTest.Fetch

Review URL: https://codereview.chromium.org/164753006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252249 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252249
Updating trunk VERSION from 1849.0 to 1850.0

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252251 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252251