Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/manifest/manifest_manager.h

Index: content/renderer/manifest/manifest_manager.h
diff --git a/content/renderer/manifest/manifest_manager.h b/content/renderer/manifest/manifest_manager.h
index 6ed61226ef3f1bdabf01fbe546adc36cfe0169c0..554617cdcbea098623ed78a86be8d3704bd8df4b 100644
--- a/content/renderer/manifest/manifest_manager.h
+++ b/content/renderer/manifest/manifest_manager.h
@@ -54,6 +54,9 @@ class ManifestManager : public RenderFrameObserver {
     ResolveStateFailure
   };
 
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   // Called when receiving a ManifestManagerMsg_RequestManifest from the browser
   // process.
   void OnHasManifest(int request_id);