Consolidate message observer API for win and aura (and touch).

chrome/browser/ui/panels/panel_browser_frame_view.cc

Index: chrome/browser/ui/panels/panel_browser_frame_view.cc
diff --git a/chrome/browser/ui/panels/panel_browser_frame_view.cc b/chrome/browser/ui/panels/panel_browser_frame_view.cc
index 9df0b784bced175987c307188d21d4febb3fc6be..f9333941c5e9b9edc97efa2e3e480df89444396c 100644
--- a/chrome/browser/ui/panels/panel_browser_frame_view.cc
+++ b/chrome/browser/ui/panels/panel_browser_frame_view.cc
@@ -176,7 +176,12 @@ void PanelBrowserFrameView::MouseWatcher::DidProcessMessage(const MSG& msg) {
       break;
   }
 }
-#else
+#elif defined(TOUCH_UI) || defined(USE_AURA)
+void PanelBrowserFrameView::MouseWatcher::DidProcessEvent(
+    const base::NativeEvent& event) {
+  NOTIMPLEMENTED();
+}
+#elif defined(TOOLKIT_USES_GTK)
 void PanelBrowserFrameView::MouseWatcher::DidProcessEvent(GdkEvent* event) {
   switch (event->type) {
     case GDK_MOTION_NOTIFY: