Passing SharedArrayBuffers over MessagePort to the same process works now See crbug.com/716320.

This works by passing a BlinkTransferableMessage over mojo, but only serializing it if the message needs to be sent across processes.

The BroadcastChannel needs to have the same behavior to support sharing SharedArrayBuffers -- currently it always serializes the BlinkCloneableMessage, which discards the SharedArrayBuffers stored on the SerializedScriptValue.