Remove unused WebMessagePortChannel::entangle() method

Chromium-side:
https://codereview.chromium.org/232623002/

BUG=361001

Review URL: https://codereview.chromium.org/232563004

git-svn-id: svn://svn.chromium.org/blink/trunk@171305 bbb929c8-8fbe-4397-9dbb-9b2b20218538