Gradient overlay for constrained window

ui/base/animation/linear_animation.h

Index: ui/base/animation/linear_animation.h
diff --git a/ui/base/animation/linear_animation.h b/ui/base/animation/linear_animation.h
index 9f385d6dc9ceccb3c5eca9d4a8f11cd99fb4db50..33f978554468dcaa2667431ed49243663a441616 100644
--- a/ui/base/animation/linear_animation.h
+++ b/ui/base/animation/linear_animation.h
@@ -32,6 +32,9 @@ class UI_EXPORT LinearAnimation : public Animation {
   // can override this to provide others.
   virtual double GetCurrentValue() const OVERRIDE;
 
+  // Change the current state of the animation to |new_value|.
+  void SetCurrentValue(double new_value);
+
   // Skip to the end of the current animation.
   void End();
 
@@ -42,13 +45,16 @@ class UI_EXPORT LinearAnimation : public Animation {
  protected:
   // Called when the animation progresses. Subclasses override this to
   // efficiently update their state.
-  virtual void AnimateToState(double state) = 0;
+  virtual void AnimateToState(double state) {}
 
   // Invoked by the AnimationContainer when the animation is running to advance
   // the animation. Use |time_now| rather than Time::Now to avoid multiple
   // animations running at the same time diverging.
   virtual void Step(base::TimeTicks time_now) OVERRIDE;
 
+  // Overriden to initialize state.
+  virtual void AnimationStarted() OVERRIDE;
+
   // Overriden to advance to the end (if End was invoked).
   virtual void AnimationStopped() OVERRIDE;