Align physical pixels of scrolled layers

cc/layer_tree_host_common.h

Index: cc/layer_tree_host_common.h
diff --git a/cc/layer_tree_host_common.h b/cc/layer_tree_host_common.h
index 8d8e5d9e64c98eaa2b170935d8e76f8f35375d02..0c998f4c513c5480ffb05eeba2df8648fc0fd34f 100644
--- a/cc/layer_tree_host_common.h
+++ b/cc/layer_tree_host_common.h
@@ -39,6 +39,11 @@ public:
     // from the given root layer (including mask and replica layers).
     template<typename LayerType> static LayerType* findLayerInSubtree(LayerType* rootLayer, int layerId);
 
+    // Content layers can be either directly scrollable or contained in an outer
+    // scrolling layer which applies the scroll transform. Given a content layer,
+    // this function returns the associated scroll layer if any.
+    template<typename LayerType> static LayerType* findScrollLayerForContentLayer(LayerType* layer);
+
     static Layer* getChildAsRawPtr(const std::vector<scoped_refptr<Layer> >& children, size_t index)
     {
         return children[index].get();
@@ -113,6 +118,21 @@ void LayerTreeHostCommon::callFunctionForSubtree(LayerType* rootLayer)
         callFunctionForSubtree<Function>(getChildAsRawPtr(rootLayer->children(), i));
 }
 
+template<typename LayerType>
+LayerType* LayerTreeHostCommon::findScrollLayerForContentLayer(LayerType* layer)
+{
+    if (!layer)
+        return 0;
+
+    if (layer->scrollable())
+        return layer;
+
+    if (layer->drawsContent() && layer->parent() && layer->parent()->scrollable())
+        return layer->parent();
+
+    return 0;
+}
+
 }  // namespace cc
 
 #endif  // CC_LAYER_TREE_HOST_COMMON_H_