Clean up MessagePortChannel/WebMessagePortChannel lifetime management

Take care of calling WebMessagePortChannel::setClient() appropriately
in MessagePortChannel's constructor and its webChannelRelease() method
rather than forcing callers to do it appropriately.

Also a few other cleanups in nearby code, and added a FIXME near some
suspicious-looking code.

R=abarth

Review URL: https://codereview.chromium.org/25560003

git-svn-id: svn://svn.chromium.org/blink/trunk@158665 bbb929c8-8fbe-4397-9dbb-9b2b20218538