Unify load/unload event state into a single enum

State tracking whether we need a load event, whether we're in a load event, whether we need an unload event, etc., is strewn across Document, DocumentLoader and FrameLoader. Centralize it in Document.

This resolves multiple outstanding FIXMEs

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22632008

git-svn-id: svn://svn.chromium.org/blink/trunk@155873 bbb929c8-8fbe-4397-9dbb-9b2b20218538