input_method: Remove an unneccessary include of mock_bus.h

BUG=234078
TEST=none

Review URL: https://codereview.chromium.org/59863006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233522 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233522