Define accelerated steps time function.

cc/animation/timing_function.h

Index: cc/animation/timing_function.h
diff --git a/cc/animation/timing_function.h b/cc/animation/timing_function.h
index cc979398e4b4b098ece44d57269c1e220605129b..9dd426ed0f9e2950217678b6b19a0c442894d6e4 100644
--- a/cc/animation/timing_function.h
+++ b/cc/animation/timing_function.h
@@ -8,6 +8,7 @@
 #include "cc/animation/animation_curve.h"
 #include "cc/base/cc_export.h"
 #include "ui/gfx/geometry/cubic_bezier.h"
+#include "ui/gfx/geometry/steps.h"
 
 namespace cc {
 
@@ -53,6 +54,28 @@ class CC_EXPORT CubicBezierTimingFunction : public TimingFunction {
   DISALLOW_ASSIGN(CubicBezierTimingFunction);
 };
 
+class CC_EXPORT StepsTimingFunction : public TimingFunction {
+ public:
+  static scoped_ptr<StepsTimingFunction> Create(int steps, bool steps_at_start);
+  virtual ~StepsTimingFunction();
+
+  virtual float GetValue(double time) const OVERRIDE;
+  virtual scoped_ptr<AnimationCurve> Clone() const OVERRIDE;
+  virtual void Range(float* min, float* max) const OVERRIDE;
+  // XXX: Should Velocity be a CubicBezierTimingFunction only? It seems to be
+  // used only for ScrollOffsetAnimationCurve. Do ScrollOffsetAnimationCurve
+  // with non bezier curves make sense?
+  virtual float Velocity(double time) const OVERRIDE;
+
+ protected:
+  StepsTimingFunction(int steps, bool steps_at_start);
+
+  gfx::Steps steps_;
+
+ private:
+  DISALLOW_ASSIGN(StepsTimingFunction);
+};
+
 class CC_EXPORT EaseTimingFunction {
  public:
   static scoped_ptr<TimingFunction> Create();