[DevTools] Only create DevToolsAgent for local main frame.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index d411147589e6a4a92c1f6315ee13c27ff4c00e2d..5872c9af4e4436f9af9e34e0acdde8474500dfb7 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -3221,8 +3221,10 @@ void RenderFrameImpl::didFinishResourceLoad(blink::WebLocalFrame* frame,
     return;
 
   // Do not show error page when DevTools is attached.
-  if (render_view_->devtools_agent_->IsAttached())
+  if (render_view_->devtools_agent_ &&
+      render_view_->devtools_agent_->IsAttached()) {
     return;
+  }
 
   // Display error page, if appropriate.
   std::string error_domain = "http";