Allow a main interstitial frame to act as an ExtensionMessagePort opener

content/browser/frame_host/render_frame_host_delegate.h

Index: content/browser/frame_host/render_frame_host_delegate.h
diff --git a/content/browser/frame_host/render_frame_host_delegate.h b/content/browser/frame_host/render_frame_host_delegate.h
index ee5aae52bda611ff4eff5537c44a373a427dae52..a2a4c599c3a08322163ffcd221b80e5d0af492b7 100644
--- a/content/browser/frame_host/render_frame_host_delegate.h
+++ b/content/browser/frame_host/render_frame_host_delegate.h
@@ -31,6 +31,7 @@ class Message;
 
 namespace content {
 class GeolocationServiceContext;
+class InterstitialPage;
 class PageState;
 class RenderFrameHost;
 class WakeLockServiceContext;
@@ -116,6 +117,10 @@ class CONTENT_EXPORT RenderFrameHostDelegate {
   // not a WebContents, returns NULL.
   virtual WebContents* GetAsWebContents();
 
+  // Returns this object cast to an InterstitialPage if it is one. Returns
+  // nullptr otherwise.
+  virtual InterstitialPage* GetAsInterstitialPage();
+
   // The render frame has requested access to media devices listed in
   // |request|, and the client should grant or deny that permission by
   // calling |callback|.