Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

chrome/renderer/prerender/prerenderer_client.h

Index: chrome/renderer/prerender/prerenderer_client.h
diff --git a/chrome/renderer/prerender/prerenderer_client.h b/chrome/renderer/prerender/prerenderer_client.h
index 134f2367c8b9cdf773244c982bfca7ab14cecb8a..f779dbfa23db89d4a47aeeb07a963b85537a149b 100644
--- a/chrome/renderer/prerender/prerenderer_client.h
+++ b/chrome/renderer/prerender/prerenderer_client.h
@@ -19,6 +19,9 @@ class PrerendererClient : public content::RenderViewObserver,
  private:
   ~PrerendererClient() override;
 
+  // RenderViewObserver implementation.
+  void OnDestruct() override;
+
   // Implements blink::WebPrerendererClient
   void willAddPrerender(blink::WebPrerender* prerender) override;
 };