Implements HandleWatcher in terms of MessagePumpMojo

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

Review URL: https://codereview.chromium.org/69883008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236954 0039d316-1c4b-4281-b951-d872f2087c98
This is the second CL of several that will eventually replace TokenService with
ProfileOAuth2TokenService. In this CL, the dependencies on TS are removed
from SigninManager and UbertokenFetcher.

BUG=305247

Review URL: https://codereview.chromium.org/71723002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236955 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236955