Adding a reverse function to the TimingFunctions and tests to prove it works.

Source/core/platform/animation/TimingFunction.h

Index: Source/core/platform/animation/TimingFunction.h
diff --git a/Source/core/platform/animation/TimingFunction.h b/Source/core/platform/animation/TimingFunction.h
index c0c478cfa15cb182f375c7dd71229d6aea38bf40..829c62a4c17afb8028ebf86caf6a70c08d9e25d2 100644
--- a/Source/core/platform/animation/TimingFunction.h
+++ b/Source/core/platform/animation/TimingFunction.h
@@ -54,6 +54,8 @@ public:
     virtual bool operator==(const TimingFunction& other) const = 0;
     virtual bool operator!=(const TimingFunction& other) const { return !operator==(other); }
 
+    virtual PassRefPtr<TimingFunction> reverse() const = 0;
+
 protected:
     TimingFunction(Type type)
         : m_type(type)
@@ -85,6 +87,11 @@ public:
         return other.type() == LinearFunction;
     }
 
+    virtual PassRefPtr<TimingFunction> reverse() const
+    {
+        return const_cast<LinearTimingFunction*>(this);
+    }
+
 private:
     LinearTimingFunction()
         : TimingFunction(LinearFunction)
@@ -184,6 +191,26 @@ public:
 
     SubType subType() const { return m_subType; }
 
+    virtual PassRefPtr<TimingFunction> reverse() const
+    {
+        switch (m_subType) {
+        case Ease:
+            return const_cast<CubicBezierTimingFunction*>(this);
+        case EaseIn:
+            return preset(EaseOut);
+        case EaseOut:
+            return preset(EaseIn);
+        case EaseInOut:
+            return const_cast<CubicBezierTimingFunction*>(this);
+        case Custom:
+            // Flip the timing function in x. We also have to flip it in y to
+            // maintain the invariant that it runs from (0, 0) to (1, 1).
+            return create(1 - m_x2, 1 - m_y2, 1 - m_x1, 1 - m_y1);
+        default:
+            ASSERT_NOT_REACHED();
+        }
+    }
+
 private:
     explicit CubicBezierTimingFunction(SubType subType, double x1, double y1, double x2, double y2)
         : TimingFunction(CubicBezierFunction)
@@ -269,6 +296,21 @@ public:
 
     SubType subType() const { return m_subType; }
 
+    virtual PassRefPtr<TimingFunction> reverse() const
+    {
+        switch (m_subType) {
+        case Start:
+            return preset(End);
+        case End:
+            return preset(Start);
+        case Custom:
+            return create(m_steps, !m_stepAtStart);
+        default:
+            ASSERT_NOT_REACHED();
+            return 0;
+        }
+    }
+
 private:
     StepsTimingFunction(SubType subType, int steps, bool stepAtStart)
         : TimingFunction(StepsFunction)
@@ -329,6 +371,17 @@ public:
         return true;
     }
 
+    virtual PassRefPtr<TimingFunction> reverse() const
+    {
+        RefPtr<ChainedTimingFunction> reversed = create();
+        for (size_t i = 0; i < m_segments.size(); i++) {
+            size_t index = m_segments.size() - i - 1;
+
+            reversed->appendSegment(1 - m_segments[index].m_min, m_segments[index].m_timingFunction->reverse().get());
+        }
+        return reversed;
+    }
+
 private:
     class Segment {
     public: