Allow a main interstitial frame to act as an ExtensionMessagePort opener

content/browser/frame_host/render_frame_host_delegate.cc

Index: content/browser/frame_host/render_frame_host_delegate.cc
diff --git a/content/browser/frame_host/render_frame_host_delegate.cc b/content/browser/frame_host/render_frame_host_delegate.cc
index ec119050bec0752ed50b1abda923561231e41f94..03d9729cac2cbd18427e4affe4b2e1f0b4ae02cf 100644
--- a/content/browser/frame_host/render_frame_host_delegate.cc
+++ b/content/browser/frame_host/render_frame_host_delegate.cc
@@ -36,6 +36,10 @@ WebContents* RenderFrameHostDelegate::GetAsWebContents() {
   return NULL;
 }
 
+InterstitialPage* RenderFrameHostDelegate::GetAsInterstitialPage() {
+  return nullptr;
+}
+
 void RenderFrameHostDelegate::RequestMediaAccessPermission(
     const MediaStreamRequest& request,
     const MediaResponseCallback& callback) {