Added notification when user presses backspace to use new shortcut.

chrome/browser/ui/cocoa/browser_window_cocoa.mm

Index: chrome/browser/ui/cocoa/browser_window_cocoa.mm
diff --git a/chrome/browser/ui/cocoa/browser_window_cocoa.mm b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
index 779a79d2a0e3d7d980d363ae85af56e5a706c75a..4a007f8bbba26a2bb8a6fe9a1029c3a95c7b1ef2 100644
--- a/chrome/browser/ui/cocoa/browser_window_cocoa.mm
+++ b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
@@ -433,6 +433,11 @@ 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.
+}
+
 LocationBar* BrowserWindowCocoa::GetLocationBar() const {
   return [controller_ locationBarBridge];
 }