Add the first unit-test that tests our IME backend.
This change is the first unit-test that tests our IME backend in the RenderWidget class. Thanks to the unit-test framework for the RenderView class created by Carlos and Brett, we can now implement this unit test. (I'm not sure I use this framework correctly, though. Please blame me if I use it improperly.)
Currently, this IME test just tests if our IME code can properly activate (or de-activate) IMEs for an <input> element and the initial position of a candidate window. I'm going to add more practical cases to this test.