Mojo: Add BindingsSupportImpl on top of HandleWatcher

Plumb MojoResult to the callback set on HandleWatcher. Revise BindingsSupport interface to make the implementation on top of HandleWatcher trivial.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/62773003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234247 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234247