DevTools: Move ForwardedInputEventHandler into the "ui" module

Now that the handler does not depend on the "sources" module, it is
only natural that we git rid of the "input" module.

R=aandrey, vsevik

Review URL: https://codereview.chromium.org/352803002

git-svn-id: svn://svn.chromium.org/blink/trunk@177001 bbb929c8-8fbe-4397-9dbb-9b2b20218538