Discard compositor frames from unloaded web content

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 4661d9816cdb894fba47df14b1f6df27a3a9d6a5..18dbf2dd0e3558201d78db878b5162a824307d21 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -305,6 +305,9 @@ class CC_EXPORT LayerTreeHost : public NON_EXPORTED_BASE(SurfaceReferenceOwner),
     return painted_device_scale_factor_;
   }
 
+  void SetContentSourceId(uint32_t);
+  uint32_t content_source_id() const { return content_source_id_; }
+
   void SetDeviceColorSpace(const gfx::ColorSpace& device_color_space);
   const gfx::ColorSpace& device_color_space() const {
     return device_color_space_;
@@ -563,6 +566,8 @@ class CC_EXPORT LayerTreeHost : public NON_EXPORTED_BASE(SurfaceReferenceOwner),
   float max_page_scale_factor_ = 1.f;
   gfx::ColorSpace device_color_space_;
 
+  uint32_t content_source_id_;
+
   SkColor background_color_ = SK_ColorWHITE;
   bool has_transparent_background_ = false;