[Mac] Don't unhide Chrome when a notification is displayed.

ui/message_center/cocoa/popup_controller.mm

 // Copyright (c) 2013 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.
 
 #import "ui/message_center/cocoa/popup_controller.h"
 
 #include <cmath>
 
 #import "base/mac/foundation_util.h"
 #import "base/mac/sdk_forward_declarations.h"
@@ skipping 203 matching lines @@
   if (isClosing_)
     [self close];
 }
 
 - (void)showWithAnimation:(NSRect)newBounds {
   bounds_ = newBounds;
   NSRect startBounds = newBounds;
   startBounds.origin.x += startBounds.size.width;
   [[self window] setFrame:startBounds display:NO];
   [[self window] setAlphaValue:0];
+  [[self window] setCanHide:NO];
   [self showWindow:nil];
 
   // Slide-in and fade-in simultaneously.
   NSDictionary* animationDict = @{
     NSViewAnimationTargetKey : [self window],
     NSViewAnimationEndFrameKey : [NSValue valueWithRect:newBounds],
     NSViewAnimationEffectKey : NSViewAnimationFadeInEffect
   };
   NSViewAnimation* animation = [self animationWithDictionary:animationDict];
   [self setBoundsAnimation:animation];
@@ skipping 55 matching lines @@
 
 - (void)mouseEntered:(NSEvent*)event {
   messageCenter_->PausePopupTimers();
 }
 
 - (void)mouseExited:(NSEvent*)event {
   messageCenter_->RestartPopupTimers();
 }
 
 @end