Remove unnecessary Platform::createMessagePortChannel method

WebMessagePortChannel::entangle() will be removed in a subsequent CL.

BUG=361001

Review URL: https://codereview.chromium.org/230083002

git-svn-id: svn://svn.chromium.org/blink/trunk@171117 bbb929c8-8fbe-4397-9dbb-9b2b20218538