cc: Make PictureLayerImpl use a better choice for animated raster scale.

cc/animation/animation_curve.h

Index: cc/animation/animation_curve.h
diff --git a/cc/animation/animation_curve.h b/cc/animation/animation_curve.h
index ca53e442d76bf188f676fc662ee0c38770ee3adb..7f38af9c6eac2fdf106532ffa1993f969c67e6bb 100644
--- a/cc/animation/animation_curve.h
+++ b/cc/animation/animation_curve.h
@@ -81,9 +81,13 @@ class CC_EXPORT TransformAnimationCurve : public AnimationCurve {
   // Returns true if this animation is a translation.
   virtual bool IsTranslation() const = 0;
 
-  // Set |max_scale| to the maximum scale along any dimension during this
-  // animation. Returns false if the maximum scale cannot be computed.
-  virtual bool MaximumScale(float* max_scale) const = 0;
+  // Set |max_scale| to the maximum scale along any dimension at the end of
+  // intermediate animation target points (eg keyframe end points). When
+  // |forward_direction| is true, the animation curve assumes it plays from
+  // the first keyframe to the last, otherwise it assumes the opposite. Returns
+  // false if the maximum scale cannot be computed.
+  virtual bool MaximumTargetScale(bool forward_direction,
+                                  float* max_scale) const = 0;
 
   // Partial Animation implementation.
   virtual CurveType Type() const override;