Mojo: Remove ProxyMessagePipeEndpoint::EnqueueMessageInternal().

We can now fold it into EnqueueMessage().

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/177003011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253337 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253337