compositor-worker: Introduce WebCompositorMutableState

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 2b6ad61a232821b77d8a034b5e41bcd863a796f9..4485ee06870a2d3c821860f2a2cfc98816e0eb49 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -5,7 +5,6 @@
 #ifndef CC_TREES_LAYER_TREE_IMPL_H_
 #define CC_TREES_LAYER_TREE_IMPL_H_
 
-#include <map>
 #include <set>
 #include <string>
 #include <vector>
@@ -135,6 +134,16 @@ class CC_EXPORT LayerTreeImpl {
 
   void PushPropertiesTo(LayerTreeImpl* tree_impl);
 
+  struct CC_EXPORT ElementLayers {
+    // Transform and opacity mutations apply to this layer.
+    LayerImpl* main = nullptr;
+    // Scroll mutations apply to this layer.
+    LayerImpl* scroll = nullptr;
+  };
+
+  void AddToElementMap(LayerImpl* layer);
+  void RemoveFromElementMap(LayerImpl* layer);
+  ElementLayers GetMutableLayers(uint64_t element_id);
   int source_frame_number() const { return source_frame_number_; }
   void set_source_frame_number(int frame_number) {
     source_frame_number_ = frame_number;
@@ -458,6 +467,8 @@ class CC_EXPORT LayerTreeImpl {
   typedef base::hash_map<int, LayerImpl*> LayerIdMap;
   LayerIdMap layer_id_map_;
 
+  base::hash_map<uint64_t, ElementLayers> element_layers_map_;
+
   // Maps from clip layer ids to scroll layer ids.  Note that this only includes
   // the subset of clip layers that act as scrolling containers.  (This is
   // derived from LayerImpl::scroll_clip_layer_ and exists to avoid O(n) walks.)