Implement frames() timing function

cc/animation/timing_function.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_ANIMATION_TIMING_FUNCTION_H_
 #define CC_ANIMATION_TIMING_FUNCTION_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "cc/animation/animation_export.h"
 #include "ui/gfx/geometry/cubic_bezier.h"
 
 namespace cc {
 
 // See http://www.w3.org/TR/css3-transitions/.
 class CC_ANIMATION_EXPORT TimingFunction {
  public:
   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;
   virtual float Velocity(double time) const = 0;
   // The smallest and largest values returned by GetValue for inputs in [0, 1].
   virtual void Range(float* min, float* max) const = 0;
   virtual std::unique_ptr<TimingFunction> Clone() const = 0;
 
  protected:
   TimingFunction();
@@ skipping 61 matching lines @@
   StepsTimingFunction(int steps, StepPosition step_position);
 
   float GetStepsStartOffset() const;
 
   int steps_;
   StepPosition step_position_;
 
   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_