Discard compositor frames from unloaded web content

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 09cd850fd801180228c5337bd2c1bea13c56f586..02c6e994d3d680dbdfe9df53c335b6183ff03d08 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -229,6 +229,9 @@ class CC_EXPORT LayerTreeImpl {
     return painted_device_scale_factor_;
   }
 
+  void set_content_source_id(uint32_t id) { content_source_id_ = id; }
+  uint32_t content_source_id() { return content_source_id_; }
+
   void SetDeviceColorSpace(const gfx::ColorSpace& device_color_space);
   const gfx::ColorSpace& device_color_space() const {
     return device_color_space_;
@@ -495,6 +498,8 @@ class CC_EXPORT LayerTreeImpl {
   float painted_device_scale_factor_;
   gfx::ColorSpace device_color_space_;
 
+  uint32_t content_source_id_;
+
   scoped_refptr<SyncedElasticOverscroll> elastic_overscroll_;
 
   std::unique_ptr<OwnedLayerImplList> layers_;