Don't show current RenderWidgetHostView while interstitial is showing.

content/browser/frame_host/navigator_delegate.h

Index: content/browser/frame_host/navigator_delegate.h
diff --git a/content/browser/frame_host/navigator_delegate.h b/content/browser/frame_host/navigator_delegate.h
index 408a06abc8fb6851254e6acefd861e68918f9457..93a3e437c9a2ff0a7a0f864554b737abfb9e6e28 100644
--- a/content/browser/frame_host/navigator_delegate.h
+++ b/content/browser/frame_host/navigator_delegate.h
@@ -127,6 +127,10 @@ class CONTENT_EXPORT NavigatorDelegate {
   // 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