Added popup that shows the new Back shortcut on Mac.

chrome/browser/ui/cocoa/browser_window_cocoa.mm

 // Copyright (c) 2012 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 "chrome/browser/ui/cocoa/browser_window_cocoa.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/logging.h"
 #import "base/mac/sdk_forward_declarations.h"
@@ skipping 416 matching lines @@
 
 bool BrowserWindowCocoa::IsFullscreen() const {
   return [controller_ isInAnyFullscreenMode];
 }
 
 bool BrowserWindowCocoa::IsFullscreenBubbleVisible() const {
   return false;  // Currently only called from toolkit-views website_settings.
 }
 
 void BrowserWindowCocoa::ShowNewBackShortcutBubble(bool forward) {
-  // TODO(mgiuca): Implement new back shortcut bubble Mac.
-  // https://crbug.com/610039.
+  ExclusiveAccessController* exclusive_access_controller =
+      [controller_ exclusiveAccessController];
+  exclusive_access_controller->ShowNewBackShortcutBubble(forward);
 }
 
 LocationBar* BrowserWindowCocoa::GetLocationBar() const {
   return [controller_ locationBarBridge];
 }
 
 void BrowserWindowCocoa::SetFocusToLocationBar(bool select_all) {
   [controller_ focusLocationBar:select_all ? YES : NO];
 }
 
@@ skipping 384 matching lines @@
 ExclusiveAccessContext* BrowserWindowCocoa::GetExclusiveAccessContext() {
   return [controller_ exclusiveAccessController];
 }
 
 void BrowserWindowCocoa::ShowImeWarningBubble(
     const extensions::Extension* extension,
     const base::Callback<void(ImeWarningBubblePermissionStatus status)>&
         callback) {
   NOTREACHED() << "The IME warning bubble is unsupported on this platform.";
 }