Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/visual_state_browsertest.cc

Index: content/renderer/visual_state_browsertest.cc
diff --git a/content/renderer/visual_state_browsertest.cc b/content/renderer/visual_state_browsertest.cc
index 26d9a6ddd26cb6ae254ec6db14fc406ed8db4fdc..2a8b49bff8fea2188eff5e7d8925cedc9b7cbc7a 100644
--- a/content/renderer/visual_state_browsertest.cc
+++ b/content/renderer/visual_state_browsertest.cc
@@ -50,6 +50,9 @@ class CommitObserver : public RenderViewObserver {
   int GetCommitCount() { return commit_count_; }
 
  private:
+  // RenderViewObserver implementation.
+  void OnDestruct() override { delete this; }
+
   std::set<base::Closure*> quit_closures_;
   int commit_count_;
 };