Send synchronous loads through the cache.

The logic for actually performing the sync request now lives in ResourceLoader.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23702040

git-svn-id: svn://svn.chromium.org/blink/trunk@158065 bbb929c8-8fbe-4397-9dbb-9b2b20218538