Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

chrome/renderer/plugins/plugin_preroller.h

Index: chrome/renderer/plugins/plugin_preroller.h
diff --git a/chrome/renderer/plugins/plugin_preroller.h b/chrome/renderer/plugins/plugin_preroller.h
index b059d77f040e78af516282d754cf9893df405c3a..3fa517fcd5a7d9d0de17e868fa9dab8f2882ef9c 100644
--- a/chrome/renderer/plugins/plugin_preroller.h
+++ b/chrome/renderer/plugins/plugin_preroller.h
@@ -44,6 +44,9 @@ class PluginPreroller : public content::PluginInstanceThrottler::Observer,
   void OnThrottleStateChange() override;
   void OnThrottlerDestroyed() override;
 
+  // content::RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   blink::WebLocalFrame* frame_;
   blink::WebPluginParams params_;
   content::WebPluginInfo info_;