1. Stop observing FSD when the delegate_ of layer_tree_view is null.

2. Stop observing FSD when the local_root_ or the client of local_root_ is null.

Once |LayerTreeView::Disconnect()| is called to shutdown the compositor, the delegate_ will be null. We should not observe FSD after that.

Change-Id: Ia0d0e27f6d7db0d2c452b8951191424884f5585a
Bug: 922980
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218230
Commit-Queue: Hongbo Song <hbsong@google.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772398}

third_party/blink/renderer/core/frame/web_frame_widget_base.cc

@@ -465,6 +465,9 @@ void WebFrameWidgetBase::DidCommitAndDrawCompositorFrame() {
 
 void WebFrameWidgetBase::DidObserveFirstScrollDelay(
     base::TimeDelta first_scroll_delay) {
+  if (!LocalRoot() || !(LocalRoot()->Client())) {
+    return;
+  }
   LocalRoot()->Client()->DidObserveFirstScrollDelay(first_scroll_delay);
 }
 

third_party/blink/renderer/platform/widget/compositing/layer_tree_view.cc

@@ -338,6 +338,9 @@ void LayerTreeView::NotifyThroughputTrackerResults(
 
 void LayerTreeView::DidObserveFirstScrollDelay(
     base::TimeDelta first_scroll_delay) {
+  if (!delegate_) {
+    return;
+  }
   delegate_->DidObserveFirstScrollDelay(first_scroll_delay);
 }