Fix bug where CSS styles are requested after node is destroyed

There's a bug which causes a crash when a node is highlighted in the
inspector and the corresponding object is destroyed in Ash. This somehow
triggers the frontend to request the styles for that node again, so
instead of NOTREACHED(), we simply return an error.

BUG=648701

Review-Url: https://codereview.chromium.org/2524863004
Cr-Commit-Position: refs/heads/master@{#434563}

ash/common/devtools/ash_devtools_css_agent.cc

@@ -64,28 +64,31 @@ AshDevToolsCSSAgent::AshDevToolsCSSAgent(AshDevToolsDOMAgent* dom_agent)
 AshDevToolsCSSAgent::~AshDevToolsCSSAgent() {}
 
 ui::devtools::protocol::Response AshDevToolsCSSAgent::getMatchedStylesForNode(
-    int nodeId,
+    int node_id,
     ui::devtools::protocol::Maybe<ui::devtools::protocol::CSS::CSSStyle>*
         inlineStyle) {
-  *inlineStyle = GetStylesForNode(nodeId);
+  *inlineStyle = GetStylesForNode(node_id);
+  if (!inlineStyle) {
+    return ui::devtools::protocol::Response::Error(
+        "Node with that id not found");
+  }
   return ui::devtools::protocol::Response::OK();
 }
 
 std::unique_ptr<ui::devtools::protocol::CSS::CSSStyle>
-AshDevToolsCSSAgent::GetStylesForNode(int nodeId) {
-  WmWindow* window = dom_agent_->GetWindowFromNodeId(nodeId);
+AshDevToolsCSSAgent::GetStylesForNode(int node_id) {
+  WmWindow* window = dom_agent_->GetWindowFromNodeId(node_id);
   if (window)
     return BuildStyles(window);
 
-  views::Widget* widget = dom_agent_->GetWidgetFromNodeId(nodeId);
+  views::Widget* widget = dom_agent_->GetWidgetFromNodeId(node_id);
   if (widget)
     return BuildStyles(widget);
 
-  views::View* view = dom_agent_->GetViewFromNodeId(nodeId);
+  views::View* view = dom_agent_->GetViewFromNodeId(node_id);
   if (view)
     return BuildStyles(view);
 
-  NOTREACHED();
   return nullptr;
 }