Allow a main interstitial frame to act as an ExtensionMessagePort opener

content/public/browser/interstitial_page.h

Index: content/public/browser/interstitial_page.h
diff --git a/content/public/browser/interstitial_page.h b/content/public/browser/interstitial_page.h
index 71b8707e54dfa88395cecda7ce7c4cd794d65e06..e22f402985dc9c73433c3ed37639055c444a369e 100644
--- a/content/public/browser/interstitial_page.h
+++ b/content/public/browser/interstitial_page.h
@@ -53,6 +53,11 @@ class InterstitialPage {
   CONTENT_EXPORT static InterstitialPage* GetInterstitialPage(
       WebContents* web_contents);
 
+  // Retrieves the InterstitialPage that hosts the RenderFrameHost, or nullptr
+  // if |rfh| is not a part of any InterstitialPage.
+  CONTENT_EXPORT static InterstitialPage* FromRenderFrameHost(
+      RenderFrameHost* rfh);
+
   virtual ~InterstitialPage() {}
 
   // Shows the interstitial page in the tab.
@@ -79,6 +84,13 @@ class InterstitialPage {
   // Sets the focus to the interstitial.
   virtual void Focus() = 0;
 
+  // Get the WebContents in which this interstitial is shown. Warning: Frames
+  // in the intersitital are NOT visible through WebContentObservers' normal
+  // notifications (e.g. RenderFrameDeleted). The only sensible use of this
+  // returned WebContents is to add a WebContentObserver and listen for the
+  // DidAttachInterstitialPage or DidDetachInterstitialPage notifications.
+  virtual WebContents* GetWebContents() const = 0;
+
   // Gets the RenderFrameHost associated with
   // the interstitial page's main frame.
   virtual RenderFrameHost* GetMainFrame() const = 0;