Use the main world script context for mime handler postMessage.

extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc

Index: extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc
diff --git a/extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc b/extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc
index 9c502565f6f0bf0d19912e2bcaa86211f381395e..f5ded41b4f148333f8906f4298f14a7ce9dcaf60 100644
--- a/extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc
+++ b/extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc
@@ -203,6 +203,8 @@ void MimeHandlerViewContainer::PostMessage(v8::Isolate* isolate,
   if (!guest_proxy_frame)
     return;
 
+  v8::Context::Scope context_scope(
+      render_frame()->GetWebFrame()->mainWorldScriptContext());
   v8::Local<v8::Object> guest_proxy_window =
       guest_proxy_frame->mainWorldScriptContext()->Global();
   gin::Dictionary window_object(isolate, guest_proxy_window);