Make HWNDMessageHandler subclass WindowImpl.

The last little bit of cleanup before HWNDMessageHandler becomes the WindowImpl.

. Removes the win32 api pass-thrus on NativeWidgetWin
. Adds win32 message pre- and post- filtering to HWNDMessageFilterDelegate so various NWW subclasses can customize message handling.
. WidgetSizeIsClientSize -> HWNDMessageHandlerDelegate
. MessageLoopForUI::Observer -> HWNDMessageHandler
. Fullscreen handler takes a HWND instead of a Widget.

Derived from: https://chromiumcodereview.appspot.com/10867096/ which is already R=sky@chromium.org

http://crbug.com/142962
TBR=sky@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=153728