Remove WebKitPoint

Intent to Deprecate and Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/pIbpN_8Lqpg/tp9f71GsslIJ

BUG=409956

Review URL: https://codereview.chromium.org/521883003

git-svn-id: svn://svn.chromium.org/blink/trunk@181252 bbb929c8-8fbe-4397-9dbb-9b2b20218538