Added notification when user presses backspace to use new shortcut.

chrome/test/base/test_browser_window.h

Index: chrome/test/base/test_browser_window.h
diff --git a/chrome/test/base/test_browser_window.h b/chrome/test/base/test_browser_window.h
index 2c75121fa8826fddc382045b31f64defb57e6434..6ea477720a9eb9e7e675a0d70ba4d02479cd7cc8 100644
--- a/chrome/test/base/test_browser_window.h
+++ b/chrome/test/base/test_browser_window.h
@@ -67,6 +67,7 @@ class TestBrowserWindow : public BrowserWindow {
   bool ShouldHideUIForFullscreen() const override;
   bool IsFullscreen() const override;
   bool IsFullscreenBubbleVisible() const override;
+  void ShowNewBackShortcutBubble(bool forward) override;
   LocationBar* GetLocationBar() const override;
   void SetFocusToLocationBar(bool select_all) override {}
   void UpdateReloadStopState(bool is_loading, bool force) override {}