Clean up EventRetargeter.h.

- Removed unused function declarations, which should be removed at r159997, from EventRetargeter.h.
- Cleaned up 'includes' and forward declarations.

BUG=None
Test=None; No behavior change;

Review URL: https://codereview.chromium.org/39533002

git-svn-id: svn://svn.chromium.org/blink/trunk@160428 bbb929c8-8fbe-4397-9dbb-9b2b20218538