Add heuristics to limit showing of new backspace UI bubble.

chrome/browser/ui/browser_window.h

Index: chrome/browser/ui/browser_window.h
diff --git a/chrome/browser/ui/browser_window.h b/chrome/browser/ui/browser_window.h
index dcbf46c8012e1bfe8677a0b364ad76bc3951a4d6..dc33811144dfbac498605de9115ee6f3dc01adf1 100644
--- a/chrome/browser/ui/browser_window.h
+++ b/chrome/browser/ui/browser_window.h
@@ -146,9 +146,13 @@ class BrowserWindow : public ui::BaseWindow {
   // Returns true if the fullscreen bubble is visible.
   virtual bool IsFullscreenBubbleVisible() const = 0;
 
-  // Shows a notice teaching the user the new shortcut for going Back or
-  // Forward.
-  virtual void ShowNewBackShortcutBubble(bool forward) = 0;
+  // Shows a notice teaching the user the new shortcut for going back or forward
+  // if the user has pressed the old shortcut more than once in three seconds
+  // and the bubble has been shown less than five times.
+  virtual void MaybeShowNewBackShortcutBubble(bool forward) = 0;
+
+  // Hides the new back shortcut bubble, if showing, by fading it out.
+  virtual void HideNewBackShortcutBubble() = 0;
 
   // Returns the size of WebContents in the browser. This may be called before
   // the TabStripModel has an active tab.