Make TransformTree::CombineTransformsBetween run faster

cc/trees/property_tree_unittest.cc

Index: cc/trees/property_tree_unittest.cc
diff --git a/cc/trees/property_tree_unittest.cc b/cc/trees/property_tree_unittest.cc
index 8eba77382654ea7f6b310e42d1582293ae3df863..e216ed4c241d02f78eb0e190b6c2ff0456826fe8 100644
--- a/cc/trees/property_tree_unittest.cc
+++ b/cc/trees/property_tree_unittest.cc
@@ -316,4 +316,42 @@ TEST(PropertyTreeTest, ComputeTransformWithSublayerScale) {
                                   transform);
 }
 
+TEST (PropertyTreeTest, FlattenOperationPropertyTest) {

[ajuma, 2015/04/16 17:34:37]

Maybe call this something like FlatteningWhenDestinationHasOnlyFlatAncestors?

+  // This test verifies the property :
+  // If A is a 2d transform, then
+  // flatten(B*A) = flatten(B)*A

[ajuma, 2015/04/16 17:34:37]

Nit: This is the property the implementation relies on, but isn't directly used
by the test itself. How about a comment along the lines of "This tests that
flattening is performed correctly when the destination and its ancestors are
flat, but the are 3d transforms and flattening between the source and
destination."?

[jaydasika, 2015/04/16 17:44:22]

The property is used by the test. But, I agree, the comment you suggested is
better than what's there right now, will make the change. 

+  TransformTree tree;
+
+  int parent = tree.Insert(TransformNode(), 0);
+  tree.Node(parent)->data.content_target_id = parent;
+  tree.Node(parent)->data.target_id = parent;
+  tree.Node(parent)->data.local.Translate(2, 2);
+
+  gfx::Transform rotation_about_x;
+  rotation_about_x.RotateAboutXAxis(15);
+
+  int child = tree.Insert(TransformNode(), parent);
+  tree.Node(child)->data.content_target_id = child;
+  tree.Node(child)->data.target_id = child;
+  tree.Node(child)->data.local = rotation_about_x;
+
+
+  int grand_child = tree.Insert(TransformNode(), child);
+  tree.Node(grand_child)->data.content_target_id = grand_child;
+  tree.Node(grand_child)->data.target_id = grand_child;
+  tree.Node(grand_child)->data.flattens_inherited_transform = true;
+
+  ComputeTransforms(&tree);
+
+  gfx::Transform flattened_rotation_about_x = rotation_about_x;
+  flattened_rotation_about_x.FlattenTo2d();
+
+  gfx::Transform grand_child_to_parent;
+  bool success =
+      tree.ComputeTransform(grand_child, parent, &grand_child_to_parent);
+  EXPECT_TRUE(success);
+  EXPECT_TRANSFORMATION_MATRIX_EQ(flattened_rotation_about_x,
+                                  grand_child_to_parent);
+}
+
 }  // namespace cc