Mojo: Add BindingsSupportImpl on top of HandleWatcher (take 2)

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

Originally reviewed at https://codereview.chromium.org/62773003/. Includes fix
to mojo.gyp to make libmojo_shell link against mojo_common_lib on Android.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/69633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234408 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234408