Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/dom_serializer_browsertest.cc

Index: content/renderer/dom_serializer_browsertest.cc
diff --git a/content/renderer/dom_serializer_browsertest.cc b/content/renderer/dom_serializer_browsertest.cc
index 3c3e549c12b6a4857c2b51ffe01707e306a00ac3..d475fe2e0daac79a39fd2f5a10517ab48fa51e7d 100644
--- a/content/renderer/dom_serializer_browsertest.cc
+++ b/content/renderer/dom_serializer_browsertest.cc
@@ -75,6 +75,9 @@ class LoadObserver : public RenderViewObserver {
   }
 
  private:
+  // RenderViewObserver implementation.
+  void OnDestruct() override { delete this; }
+
   base::Closure quit_closure_;
 };