Add heuristics to limit showing of new backspace UI bubble.

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 cd07be82d2e9fcd0900556abc65108fd6e3c3095..e576204bec4c1fc9eee51fa8e14c64eb3ae26bb3 100644
--- a/chrome/test/base/test_browser_window.h
+++ b/chrome/test/base/test_browser_window.h
@@ -67,7 +67,8 @@ class TestBrowserWindow : public BrowserWindow {
   bool ShouldHideUIForFullscreen() const override;
   bool IsFullscreen() const override;
   bool IsFullscreenBubbleVisible() const override;
-  void ShowNewBackShortcutBubble(bool forward) override;
+  void MaybeShowNewBackShortcutBubble(bool forward) override {}
+  void HideNewBackShortcutBubble() override {}
   LocationBar* GetLocationBar() const override;
   void SetFocusToLocationBar(bool select_all) override {}
   void UpdateReloadStopState(bool is_loading, bool force) override {}