When a Resource is reused, default to sending cached data asynchronously.

Previously, we had defaulted to sending data synchronously, but had a separate
timer for each reuse of a RawResource. This patches makes the default
asychronous, and uses a single timer for all reuses across all Resources.

Currently, there are approximately 25 tests that expect data to be returned
synchronously (for images, scripts, and css stylesheets), so those types still
return data synchronously. We should work on moving those to async delivery.

BUG=237366

Review URL: https://chromiumcodereview.appspot.com/16365011

git-svn-id: svn://svn.chromium.org/blink/trunk@156328 bbb929c8-8fbe-4397-9dbb-9b2b20218538