Replace all WebCore::MessagePortChannel usage with blink::WebMessagePortChannel

This class may have made sense in WebKit, but in Blink it's nothing but a fancy
smart pointer around the actual platform abstraction.

MessagePortChannel's code has either been moved into its callers (createChannel,
postMessageToRemote, tryGetMessageFromRemote) or replaced by existing code (OwnPtr,
OwnedPtrDeleter).

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/73223003

git-svn-id: svn://svn.chromium.org/blink/trunk@162054 bbb929c8-8fbe-4397-9dbb-9b2b20218538