Add support for draggable regions (issue #1645).

Regions are defined using the '-webkit-app-region: drag/no-drag'
CSS property and passed to the CefDragHandler::
OnDraggableRegionsChanged callback.

tests/cefclient/browser/root_window_gtk.h

@@ -58,6 +58,8 @@ class RootWindowGtk : public RootWindow,
   void OnSetLoadingState(bool isLoading,
                          bool canGoBack,
                          bool canGoForward) OVERRIDE;
+  void OnSetDraggableRegions(
+      const std::vector<CefDraggableRegion>& regions) OVERRIDE;
 
   void NotifyDestroyedIfDone();