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..868d458de8aafe078940743a1abdda67b24ebeb5 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
+  // notificaions (e.g. RenderFrameDeleted). The only sensible use of this

[Charlie Reis, 2016/01/26 01:14:30]

nit: notifications

[robwu, 2016/01/26 11:29:32]

Done.

+  // returned WebContents is to add a WebContentObserver and listen for the
+  // DidAttachInterstitialPage or DidDetachInterstitialPage notifications.
+  virtual WebContents* GetUnderlyingWebContents() const = 0;

[Charlie Reis, 2016/01/26 01:14:30]

Underlying isn't a term we use elsewhere for interstitials and WebContents. 
Let's stick with GetWebContents() now that you have the nice warning comment.

[robwu, 2016/01/26 11:29:32]

Done. (I called it GetUnderlyingWebContents because the class already had an
UnderlyingContentObserver, and it seemed like a good choice for a unique name.)

[Charlie Reis, 2016/01/26 23:58:27]

Ah, I was mistaken.  But yeah, that's hidden away in the implementation file, so
I like the simpler name here in the public API.

+
   // Gets the RenderFrameHost associated with
   // the interstitial page's main frame.
   virtual RenderFrameHost* GetMainFrame() const = 0;