Remove non-Mac WebInputEventFactory and KeyCodeConversion.

These are unused once this Chromium CL lands:
https://codereview.chromium.org/237963015/

BUG=363455

Review URL: https://codereview.chromium.org/241023003

git-svn-id: svn://svn.chromium.org/blink/trunk@171898 bbb929c8-8fbe-4397-9dbb-9b2b20218538