Make IPC::Channel::Listener:OnMessageReceived have a return value indicating whether a message was processed or not.

TBR=brettw
Review URL: http://codereview.chromium.org/5978003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@70139 0039d316-1c4b-4281-b951-d872f2087c98