Mojo: Add a mojo::system::MessageQueue.

I'll "need" this to transfer messages from a LocalMessagePipeEndpoint to
a ProxyMessagePipeEndpoint.

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/176763007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254600 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254600