Do not attempt to detect transfering the 'target' MessagePort in postMessage

The attempt results in a memory leak in Blink (bug 311229), yet fails to do
a through job of protecting against API abuse (bug 312962). Safari fails
similarly.

Removing MessagePortChannel::m_entangledChannel allows MessagePortChannel to stop being
refcounted and is instead managed via OwnPtrs.

I've filed a spec bug against HTML at https://www.w3.org/Bugs/Public/show_bug.cgi?id=23685,
and from IRC discussion with the editor it seems likely that the "throw" requirement
will be removed.

R=abarth@chromium.org
BUG=311229,312962

Review URL: https://codereview.chromium.org/46303007

git-svn-id: svn://svn.chromium.org/blink/trunk@161083 bbb929c8-8fbe-4397-9dbb-9b2b20218538