Introduce InputMethodController class to be a place of input method related code

This patch introduces InputMethodController class to be a place of input method related code for ease of maintenance, and CompositionUnderline.h to reduce #include dependency.

This patch moves code from Editor class and InputMethodContext class into InputMethodController. Further patches will move more codes.

BUG=n/a
TEST=n/a; no behavior changes
R=kochi@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/22527006

git-svn-id: svn://svn.chromium.org/blink/trunk@155828 bbb929c8-8fbe-4397-9dbb-9b2b20218538