Discard compositor frames from unloaded web content

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index c0425180ad777b2a0bfe41e6dda4852e1f7b20a1..2203ab25bd24f4374009f2e759e7ae1b20f6bf8a 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -101,6 +101,7 @@ LayerTreeHost::LayerTreeHost(InitParams* params, CompositorMode mode)
       debug_state_(settings_.initial_debug_state),
       id_(s_layer_tree_host_sequence_number.GetNext() + 1),
       task_graph_runner_(params->task_graph_runner),
+      content_source_id_(0),
       event_listener_properties_(),
       mutator_host_(params->mutator_host),
       image_worker_task_runner_(params->image_worker_task_runner) {
@@ -961,6 +962,13 @@ void LayerTreeHost::SetDeviceColorSpace(
       this, [](Layer* layer) { layer->SetNeedsDisplay(); });
 }
 
+void LayerTreeHost::SetContentSourceId(uint32_t id) {
+  if (content_source_id_ == id)
+    return;
+  content_source_id_ = id;
+  SetNeedsCommit();
+}
+
 void LayerTreeHost::RegisterLayer(Layer* layer) {
   DCHECK(!LayerById(layer->id()));
   DCHECK(!in_paint_layer_contents_);
@@ -1131,6 +1139,8 @@ void LayerTreeHost::PushPropertiesTo(LayerTreeImpl* tree_impl) {
 
   tree_impl->SetDeviceColorSpace(device_color_space_);
 
+  tree_impl->set_content_source_id(content_source_id_);
+
   if (pending_page_scale_animation_) {
     tree_impl->SetPendingPageScaleAnimation(
         std::move(pending_page_scale_animation_));