MacViews: Add WidgetEventGenerator to abstract platform-specific event generation for tests.

ui/views/widget/native_widget_mac.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/widget/native_widget_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/mac/foundation_util.h"
 #include "base/mac/scoped_nsobject.h"
@@ skipping 571 matching lines @@
 }
 
 // static
 void NativeWidgetPrivate::ReparentNativeView(gfx::NativeView native_view,
                                              gfx::NativeView new_parent) {
   NOTIMPLEMENTED();
 }
 
 // static
 bool NativeWidgetPrivate::IsMouseButtonDown() {
-  NOTIMPLEMENTED();
-  return false;
+  return [NSEvent pressedMouseButtons] != 0;
 }
 
 // static
 gfx::FontList NativeWidgetPrivate::GetWindowTitleFontList() {
   NOTIMPLEMENTED();
   return gfx::FontList();
 }
 
 }  // namespace internal
 }  // namespace views