Don't show current RenderWidgetHostView while interstitial is showing.

content/browser/frame_host/navigator_delegate.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_FRAME_HOST_NAVIGATOR_DELEGATE_H_
 #define CONTENT_BROWSER_FRAME_HOST_NAVIGATOR_DELEGATE_H_
 
 #include "base/strings/string16.h"
 #include "content/public/browser/invalidate_type.h"
 #include "content/public/browser/navigation_controller.h"
@@ skipping 109 matching lines @@
   virtual std::vector<std::unique_ptr<NavigationThrottle>>
   CreateThrottlesForNavigation(NavigationHandle* navigation_handle);
 
   // PlzNavigate
   // Called at the start of the navigation to get opaque data the embedder
   // wants to see passed to the corresponding URLRequest on the IO thread.
   // In the case of a navigation to an interstitial, no call will be made to the
   // embedder and |nullptr| is returned.
   virtual std::unique_ptr<NavigationUIData> GetNavigationUIData(
       NavigationHandle* navigation_handle);
+
+  // Whether the delegate is displaying an interstitial page over the current
+  // page.
+  virtual bool ShowingInterstitialPage() const = 0;
 };
 
 }  // namspace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_NAVIGATOR_DELEGATE_H_