Added popup that shows the new Back shortcut on Mac.

chrome/browser/ui/cocoa/browser/exclusive_access_controller_views.mm

Index: chrome/browser/ui/cocoa/browser/exclusive_access_controller_views.mm
diff --git a/chrome/browser/ui/cocoa/browser/exclusive_access_controller_views.mm b/chrome/browser/ui/cocoa/browser/exclusive_access_controller_views.mm
index 50a20aac825e71c135ab527dbcffca2214922fe8..df37742bb1fe7934cf430f1c9d62a1745d12e123 100644
--- a/chrome/browser/ui/cocoa/browser/exclusive_access_controller_views.mm
+++ b/chrome/browser/ui/cocoa/browser/exclusive_access_controller_views.mm
@@ -13,6 +13,7 @@
 #include "chrome/browser/ui/exclusive_access/exclusive_access_manager.h"
 #include "chrome/browser/ui/status_bubble.h"
 #include "chrome/browser/ui/views/exclusive_access_bubble_views.h"
+#include "chrome/browser/ui/views/new_back_shortcut_bubble.h"
 #include "chrome/common/pref_names.h"
 #include "components/prefs/pref_service.h"
 #include "ui/base/cocoa/cocoa_base_utils.h"
@@ -50,6 +51,13 @@ void ExclusiveAccessController::Show() {
   [cocoa_bubble_ showWindow];
 }
 
+void ExclusiveAccessController::ShowNewBackShortcutBubble(bool forward) {
+  if (new_back_shortcut_bubble_)
+    new_back_shortcut_bubble_->UpdateContent(forward);
+  else
+    new_back_shortcut_bubble_.reset(new NewBackShortcutBubble(this, forward));
+}
+
 void ExclusiveAccessController::Destroy() {
   views_bubble_.reset();
   [cocoa_bubble_ closeImmediately];