Create NavigationHandles for interstitials if needed

content/browser/frame_host/navigation_handle_impl.h

Index: content/browser/frame_host/navigation_handle_impl.h
diff --git a/content/browser/frame_host/navigation_handle_impl.h b/content/browser/frame_host/navigation_handle_impl.h
index f6059d71e0ea72f4953cef9191bc9826507304b1..18d12057910e42cb264d6d58d7e967fbfa0d6d29 100644
--- a/content/browser/frame_host/navigation_handle_impl.h
+++ b/content/browser/frame_host/navigation_handle_impl.h
@@ -126,6 +126,16 @@ class CONTENT_EXPORT NavigationHandleImpl : public NavigationHandle {
 
   NavigationData* GetNavigationData() override;
 
+  // The NavigatorDelegate to notify/query for various navigation events.
+  // Normally this is the WebContents, except if this NavigationHandle was
+  // created during a navigation to an interstitial page. In this case it will
+  // be the InterstitialPage itself.
+  //
+  // Note: due to the interstitial navigation case, all calls that can possibly
+  // expose the NavigationHandle to code outside of content/ MUST go though the
+  // NavigatorDelegate. In particular, the ContentBrowserClient should not be
+  // called directly form the NavigationHandle code. Thus, these calls will not
+  // expose the NavigationHandle when navigating to an InterstialPage.
   NavigatorDelegate* GetDelegate() const;
 
   RequestContextType GetRequestContextType() const;