[cc] Apply sublayer transform about anchor instead of about layer center.

cc/layer_tree_host_common.cc

Index: cc/layer_tree_host_common.cc
diff --git a/cc/layer_tree_host_common.cc b/cc/layer_tree_host_common.cc
index e316c370fad9a7c6f3423e0b97c60f7764cd2398..4d3377a919de45164bb5a33f7b0663b8d4397fcd 100644
--- a/cc/layer_tree_host_common.cc
+++ b/cc/layer_tree_host_common.cc
@@ -169,6 +169,7 @@ static inline bool transformToParentIsKnown(LayerImpl*)
 
 static inline bool transformToParentIsKnown(Layer* layer)
 {
+
     return !layer->transformIsAnimating();
 }
 
@@ -570,15 +571,12 @@ static void calculateDrawPropertiesInternal(LayerType* layer, const gfx::Transfo
     //        Tr[origin2anchor] is the translation from the layer's origin to its anchor point
     //        Tr[origin2center] is the translation from the layer's origin to its center
     //        M[layer] is the layer's matrix (applied at the anchor point)
-    //        M[sublayer] is the layer's sublayer transform (applied at the layer's center)
+    //        M[sublayer] is the layer's sublayer transform (also applied at the layer's anchor point)
     //        S[layer2content] is the ratio of a layer's contentBounds() to its bounds().
     //
     //    Some composite transforms can help in understanding the sequence of transforms:
     //        compositeLayerTransform = Tr[origin2anchor] * M[layer] * Tr[origin2anchor].inverse()
-    //        compositeSublayerTransform = Tr[origin2center] * M[sublayer] * Tr[origin2center].inverse()
-    //
-    //    In words, the layer transform is applied about the anchor point, and the sublayer transform is
-    //    applied about the center of the layer.
+    //        compositeSublayerTransform = Tr[origin2anchor] * M[sublayer] * Tr[origin2anchor].inverse()
     //
     // 4. When a layer (or render surface) is drawn, it is drawn into a "target render surface". Therefore the draw
     //    transform does not necessarily transform from screen space to local layer space. Instead, the draw transform
@@ -754,7 +752,7 @@ static void calculateDrawPropertiesInternal(LayerType* layer, const gfx::Transfo
         layerDrawProperties.target_space_transform.Scale(renderSurfaceSublayerScale.x() / layer->contentsScaleX(), renderSurfaceSublayerScale.y() / layer->contentsScaleY());
 
         // Inside the surface's subtree, we scale everything to the owning layer's scale.
-        // The sublayer matrix transforms centered layer rects into target
+        // The sublayer matrix transforms layer rects into target
         // surface content space.
         DCHECK(sublayerMatrix.IsIdentity());
         sublayerMatrix.Scale(renderSurfaceSublayerScale.x(), renderSurfaceSublayerScale.y());
@@ -867,11 +865,11 @@ static void calculateDrawPropertiesInternal(LayerType* layer, const gfx::Transfo
     if (!layer->preserves3D())
         sublayerMatrix.FlattenTo2d();
 
-    // Apply the sublayer transform at the center of the layer.
+    // Apply the sublayer transform at the anchor point of the layer.
     if (!layer->sublayerTransform().IsIdentity()) {
-        sublayerMatrix.Translate(0.5 * bounds.width(), 0.5 * bounds.height());
+        sublayerMatrix.Translate(layer->anchorPoint().x() * bounds.width(), layer->anchorPoint().y() * bounds.height());
         sublayerMatrix.PreconcatTransform(layer->sublayerTransform());
-        sublayerMatrix.Translate(-0.5 * bounds.width(), -0.5 * bounds.height());
+        sublayerMatrix.Translate(-layer->anchorPoint().x() * bounds.width(), -layer->anchorPoint().y() * bounds.height());
     }
 
     LayerList& descendants = (layer->renderSurface() ? layer->renderSurface()->layerList() : layerList);