Rename IBusBridge to IMEBridge, and IBusEngineHandlerInterface to IMEEngineHandlerInterface

This code is about IME Engine singleton and IBus implementation no
longer exists.

BUG=283612

Review URL: https://codereview.chromium.org/134993003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247191 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247191