Fix context menu for sandbox devtools (#11933)

[alexeykuzmin]

#11933 to 1-8-x

[alexeykuzmin]

@nitsakh Can you please review it?
I got conflicts chert-picking it to 1-8-x, and had to remove this piece:

-  // Only allow preload for the main frame
-  if (!render_frame->IsMainFrame())
+  // Only allow preload for the main frame or
+  // For devtools we still want to run the preload_bundle script
+  if (!render_frame->IsMainFrame() && !IsDevTools(render_frame))
     return;

Because these lines are absent in 1-8-x:

if (!render_frame->IsMainFrame())
  return

[nitsakh]

👍