Move synchronously fetching to ResourceFetcher, as well as most of the logic
for synthesizing callbacks for memory cache hits.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/24009002

git-svn-id: svn://svn.chromium.org/blink/trunk@157384 bbb929c8-8fbe-4397-9dbb-9b2b20218538