Skip requests for test callbacks from the layer tree view if it is closed.

Bug: 851647
Change-Id: Id7acf348daf9a2af3a961a59643593b40da686a3
Reviewed-on: https://chromium-review.googlesource.com/1097227Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566529}

third_party/blink/renderer/core/exported/web_view_impl.cc

@@ -1850,12 +1850,14 @@ void WebViewImpl::PaintIgnoringCompositing(WebCanvas* canvas,
 #endif
 
 void WebViewImpl::LayoutAndPaintAsync(base::OnceClosure callback) {
-  layer_tree_view_->LayoutAndPaintAsync(std::move(callback));
+  if (layer_tree_view_)
+    layer_tree_view_->LayoutAndPaintAsync(std::move(callback));
 }
 
 void WebViewImpl::CompositeAndReadbackAsync(
     base::OnceCallback<void(const SkBitmap&)> callback) {
-  layer_tree_view_->CompositeAndReadbackAsync(std::move(callback));
+  if (layer_tree_view_)
+    layer_tree_view_->CompositeAndReadbackAsync(std::move(callback));
 }
 
 void WebViewImpl::ThemeChanged() {