Discard compositor frames from unloaded web content

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index dad33f43e9b245dbb29e6d1c19069a06e3dbefe1..e6140a90982dc418289f7c662347c255cf7f4d4e 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -73,6 +73,7 @@ LayerTreeImpl::LayerTreeImpl(
       max_page_scale_factor_(0),
       device_scale_factor_(1.f),
       painted_device_scale_factor_(1.f),
+      content_source_id_(0),
       elastic_overscroll_(elastic_overscroll),
       layers_(new OwnedLayerImplList),
       viewport_size_invalid_(false),
@@ -470,6 +471,8 @@ void LayerTreeImpl::PushPropertiesTo(LayerTreeImpl* target_tree) {
   target_tree->SetDeviceColorSpace(device_color_space_);
   target_tree->elastic_overscroll()->PushPendingToActive();
 
+  target_tree->set_content_source_id(content_source_id());
+
   target_tree->pending_page_scale_animation_ =
       std::move(pending_page_scale_animation_);