Enable sending MessagePorts to a different renderer (Blink side).

The corresponding Chromium side is: https://codereview.chromium.org/24733002/

(Tests are included in the Chromium CL.)

BUG=278336

Review URL: https://codereview.chromium.org/24739002

git-svn-id: svn://svn.chromium.org/blink/trunk@159773 bbb929c8-8fbe-4397-9dbb-9b2b20218538