MacViews: Implement NativeWidgetMac::SetOpacity function.

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 464 matching lines @@
     return;
 
   bridge_->ToggleDesiredFullscreenState();
 }
 
 bool NativeWidgetMac::IsFullscreen() const {
   return bridge_ && bridge_->target_fullscreen_state();
 }
 
 void NativeWidgetMac::SetOpacity(unsigned char opacity) {
-  NOTIMPLEMENTED();
+  [GetNativeWindow() setAlphaValue:opacity / 255.0];
 }
 
 void NativeWidgetMac::SetUseDragFrame(bool use_drag_frame) {
   NOTIMPLEMENTED();
 }
 
 void NativeWidgetMac::FlashFrame(bool flash_frame) {
   NOTIMPLEMENTED();
 }
 
@@ skipping 225 matching lines @@
   [[ViewsNSWindowCloseAnimator alloc] initWithWindow:window];
 }
 
 - (void)animationDidEnd:(NSAnimation*)animation {
   [window_ close];
   [animation_ setDelegate:nil];
   [self release];
 }
 
 @end