Implement frames() timing 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 0f475e7624a1a2f82c0c3aed3a4ac8fa1bf090b7..5ff8d3561bd82e3f174d2a1ee7879f309ae77269 100644
--- a/cc/animation/timing_function.h
+++ b/cc/animation/timing_function.h
@@ -19,7 +19,7 @@ class CC_ANIMATION_EXPORT TimingFunction {
   virtual ~TimingFunction();
 
   // Note that LINEAR is a nullptr TimingFunction (for now).
-  enum class Type { LINEAR, CUBIC_BEZIER, STEPS };
+  enum class Type { LINEAR, CUBIC_BEZIER, STEPS, FRAMES };
 
   virtual Type GetType() const = 0;
   virtual float GetValue(double t) const = 0;
@@ -101,6 +101,29 @@ class CC_ANIMATION_EXPORT StepsTimingFunction : public TimingFunction {
   DISALLOW_ASSIGN(StepsTimingFunction);
 };
 
+class CC_ANIMATION_EXPORT FramesTimingFunction : public TimingFunction {
+ public:
+  static std::unique_ptr<FramesTimingFunction> Create(int frames);
+  ~FramesTimingFunction() override;
+
+  // TimingFunction implementation.
+  Type GetType() const override;
+  float GetValue(double t) const override;
+  std::unique_ptr<TimingFunction> Clone() const override;
+  void Range(float* min, float* max) const override;
+  float Velocity(double time) const override;
+
+  int frames() const { return frames_; }
+  double GetPreciseValue(double t) const;
+
+ private:
+  explicit FramesTimingFunction(int frames);
+
+  int frames_;
+
+  DISALLOW_ASSIGN(FramesTimingFunction);
+};
+
 }  // namespace cc
 
 #endif  // CC_ANIMATION_TIMING_FUNCTION_H_