Combine MessagePortChannel and PlatformMessagePortChannel into one class

These were historically separated due to WebKit layering concerns.
In Blink, they can be a single, ThreadSafeRefCounted class.

Also get rid of no-op locallyEntangledPort() method which always returned null.

R=abarth

Review URL: https://codereview.chromium.org/25321003

git-svn-id: svn://svn.chromium.org/blink/trunk@158577 bbb929c8-8fbe-4397-9dbb-9b2b20218538