Adds AnimationContainer, which can be used to group a set of

app/throb_animation.h

Index: app/throb_animation.h
diff --git a/app/throb_animation.h b/app/throb_animation.h
index 43e41979d87f3667f96d69e69438db5d35dd4551..2405724151f43932ee688fd68afc1e4b67c329d1 100644
--- a/app/throb_animation.h
+++ b/app/throb_animation.h
@@ -31,9 +31,6 @@ class ThrobAnimation : public SlideAnimation {
   virtual void Show();
   virtual void Hide();
 
-  // Overriden to continually throb (assuming we're throbbing).
-  virtual void Step();
-
   // Overridden to maintain the slide duration.
   virtual void SetSlideDuration(int duration) { slide_duration_ = duration; }
 
@@ -41,6 +38,10 @@ class ThrobAnimation : public SlideAnimation {
   void set_cycles_remaining(int value) { cycles_remaining_ = value; }
   int cycles_remaining() const { return cycles_remaining_; }
 
+ protected:
+  // Overriden to continually throb (assuming we're throbbing).
+  virtual void Step(base::TimeTicks time_now);
+
  private:
   // Resets state such that we behave like SlideAnimation.
   void ResetForSlide();