Prevent crash in InspectorLayerTreeAgent when the root layer is nullptr

Bug: 1011914
Change-Id: I2978b8bd6c7ee94c133b4a133c9df311382fc963
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377610Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801922}

third_party/blink/renderer/core/inspector/inspector_layer_tree_agent.cc

@@ -348,6 +348,8 @@ const cc::Layer* InspectorLayerTreeAgent::RootLayer() {
 }
 
 static const cc::Layer* FindLayerById(const cc::Layer* root, int layer_id) {
+  if (!root)
+    return nullptr;
   if (root->id() == layer_id)
     return root;
   for (auto child : root->children()) {