Copy Editor.{cpp,h} into CompositionUnderline.h and InputMethodController.{cpp,h}

This patch introduces three files
 - CompositionUnderline.h
 - InputMethodController.cpp
 - InputMethodController.h
as copy of Editor.{cpp.h} for preparation of moving IME related code out from Editor.cpp to reduce file size of Editor class for ease of maintenance.

BUG=n/a
TEST=n/a
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/22670002

git-svn-id: svn://svn.chromium.org/blink/trunk@155746 bbb929c8-8fbe-4397-9dbb-9b2b20218538