Align physical pixels of scrolled layers

cc/layer_tree_host_common.h

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_LAYER_TREE_HOST_COMMON_H_
 #define CC_LAYER_TREE_HOST_COMMON_H_
 
 #include "base/memory/ref_counted.h"
 #include "cc/cc_export.h"
 #include "cc/scoped_ptr_vector.h"
@@ skipping 21 matching lines @@
     static bool layerHasTouchEventHandlersAt(const gfx::PointF& screenSpacePoint, LayerImpl* layerImpl);
 
     template<typename LayerType> static bool renderSurfaceContributesToTarget(LayerType*, int targetSurfaceLayerID);
 
     template<class Function, typename LayerType> static void callFunctionForSubtree(LayerType* rootLayer);
 
     // Returns a layer with the given id if one exists in the subtree starting
     // 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();
     }
 
     static LayerImpl* getChildAsRawPtr(const ScopedPtrVector<LayerImpl>& children, size_t index)
     {
         return children[index];
     }
 
@@ skipping 54 matching lines @@
     if (LayerType* replicaLayer = rootLayer->replicaLayer()) {
         Function()(replicaLayer);
         if (LayerType* maskLayer = replicaLayer->maskLayer())
             Function()(maskLayer);
     }
 
     for (size_t i = 0; i < rootLayer->children().size(); ++i)
         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_