Allow a main interstitial frame to act as an ExtensionMessagePort opener

content/browser/frame_host/interstitial_page_impl.cc

Index: content/browser/frame_host/interstitial_page_impl.cc
diff --git a/content/browser/frame_host/interstitial_page_impl.cc b/content/browser/frame_host/interstitial_page_impl.cc
index 9e63d829f1ae2fa5b32dbb9a8aeae248441e0e42..866a8af5118d3f8a923c46e4b6196d2b9987922e 100644
--- a/content/browser/frame_host/interstitial_page_impl.cc
+++ b/content/browser/frame_host/interstitial_page_impl.cc
@@ -148,6 +148,14 @@ InterstitialPage* InterstitialPage::GetInterstitialPage(
   return iter->second;
 }
 
+InterstitialPage* InterstitialPage::FromRenderFrameHost(RenderFrameHost* rfh) {
+  if (!rfh)
+    return nullptr;
+  return static_cast<RenderFrameHostImpl*>(rfh)
+      ->delegate()
+      ->GetAsInterstitialPage();
+}
+
 InterstitialPageImpl::InterstitialPageImpl(
     WebContents* web_contents,
     RenderWidgetHostDelegate* render_widget_host_delegate,
@@ -424,6 +432,10 @@ void InterstitialPageImpl::UpdateTitle(
   controller_->delegate()->NotifyNavigationStateChanged(INVALIDATE_TYPE_TITLE);
 }
 
+InterstitialPage* InterstitialPageImpl::GetAsInterstitialPage() {
+  return this;
+}
+
 AccessibilityMode InterstitialPageImpl::GetAccessibilityMode() const {
   if (web_contents_)
     return static_cast<WebContentsImpl*>(web_contents_)->GetAccessibilityMode();