Mojo: Remove incorrect DCHECK in ProxyMessagePipeEndpoint::EnqueueMessage().

It's clearly wrong, since the (correct) code in Run() contradicts the
assertion. Add a test to check this.

(I ran into this while testing the out-of-process case. Ha.)

R=davemoore@chromium.org

Review URL: https://codereview.chromium.org/108813006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240202 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240202