Add type casting functions for subclasses of Entry/EntrySync, and use them.

This CL helps to detect bad casts.

BUG=309516

Review URL: https://codereview.chromium.org/112393002

git-svn-id: svn://svn.chromium.org/blink/trunk@163676 bbb929c8-8fbe-4397-9dbb-9b2b20218538