Adds classes to support WebUI mojo bindings

Nothing is wired up yet, nor is any of created yet. But this is the
necessary plumbing.

BUG=none
TEST=none
R=darin@chromium.org

Review URL: https://codereview.chromium.org/190853006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256772 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256772