Remove Entity interface

Intent to Remove:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/k3tZGP2EANc

BUG=338602

Review URL: https://codereview.chromium.org/143673005

git-svn-id: svn://svn.chromium.org/blink/trunk@165953 bbb929c8-8fbe-4397-9dbb-9b2b20218538