Implement frames() timing function

cc/animation/timing_function.cc

Index: cc/animation/timing_function.cc
diff --git a/cc/animation/timing_function.cc b/cc/animation/timing_function.cc
index b7308bb31c9564fa588a296172a640a631ac70e0..f2960838f6e46a3259033bea08384872121690d8 100644
--- a/cc/animation/timing_function.cc
+++ b/cc/animation/timing_function.cc
@@ -127,4 +127,39 @@ float StepsTimingFunction::GetStepsStartOffset() const {
   }
 }
 
+std::unique_ptr<FramesTimingFunction> FramesTimingFunction::Create(int frames) {
+  return base::WrapUnique(new FramesTimingFunction(frames));
+}
+
+FramesTimingFunction::FramesTimingFunction(int frames) : frames_(frames) {}
+
+FramesTimingFunction::~FramesTimingFunction() {}
+
+TimingFunction::Type FramesTimingFunction::GetType() const {
+  return Type::FRAMES;
+}
+
+float FramesTimingFunction::GetValue(double t) const {
+  return static_cast<float>(GetPreciseValue(t));
+}
+
+std::unique_ptr<TimingFunction> FramesTimingFunction::Clone() const {
+  return base::WrapUnique(new FramesTimingFunction(*this));
+}
+
+void FramesTimingFunction::Range(float* min, float* max) const {
+  *min = 0.0f;
+  *max = 1.0f;
+}
+
+float FramesTimingFunction::Velocity(double x) const {
+  return 0.0f;
+}
+
+double FramesTimingFunction::GetPreciseValue(double t) const {
+  const double frames = static_cast<double>(frames_);
+  return MathUtil::ClampToRange(std::floor(frames * t) / (frames - 1), 0.0,
+                                1.0);
+}
+
 }  // namespace cc