Add support for inverse transform animations.

ui/compositor/layer_animation_element.h

Index: ui/compositor/layer_animation_element.h
diff --git a/ui/compositor/layer_animation_element.h b/ui/compositor/layer_animation_element.h
index c5949d35e2120a48ef112996622f6159e00f1bba..3af1847e866aefac991b0e164fabcf5cff0e5e9e 100644
--- a/ui/compositor/layer_animation_element.h
+++ b/ui/compositor/layer_animation_element.h
@@ -65,6 +65,15 @@ class COMPOSITOR_EXPORT LayerAnimationElement {
       const gfx::Transform& transform,
       base::TimeDelta duration);
 
+  // Creates an element that counters a transition to the given transform. This
+  // should be used in tandem with an element created with
+  // CreateTransformElement with the same parameters. The caller owns the return
+  // value.
+  static LayerAnimationElement* CreateInverseTransformElement(
+      const gfx::Transform& parent_start,
+      const gfx::Transform& parent_end,
+      base::TimeDelta duration);
+
   // Creates an element that transitions to another in a way determined by an
   // interpolated transform. The element accepts ownership of the interpolated
   // transform. NB: at every step, the interpolated transform clobbers the