Fixes lack of forward declaration of GdkEvent in render_widget_host_view.

This fixes bot redness caused by 10411086

Bug=None
Test=Bots go green

Review URL: https://chromiumcodereview.appspot.com/10421030

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@138418 0039d316-1c4b-4281-b951-d872f2087c98

content/public/browser/render_widget_host_view.h

@@ -16,6 +16,10 @@
 #include "third_party/WebKit/Source/WebKit/chromium/public/WebInputEvent.h"
 #include "ui/gfx/native_widget_types.h"
 
+#if defined(TOOLKIT_GTK)
+class GdkEventButton;
+#endif
+
 class BrowserAccessibilityManager;
 
 namespace gfx {