Mojo: Get rid of LocalMessagePipeEndpoint::MessageQueueEntry.

Instead, add the ability to attach dispatchers directly to a
MessageInTransit, and just use a queue of MessageInTransits.

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/177713007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253249 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253249