Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/public/test/frame_load_waiter.h

Index: content/public/test/frame_load_waiter.h
diff --git a/content/public/test/frame_load_waiter.h b/content/public/test/frame_load_waiter.h
index d3f8ac8b33886d4e62d2b8c94e426748a50c82e7..155f92ea17f73dbe2af88e0e2fc89f8b7dfae122 100644
--- a/content/public/test/frame_load_waiter.h
+++ b/content/public/test/frame_load_waiter.h
@@ -20,7 +20,9 @@ class FrameLoadWaiter : public RenderFrameObserver {
   void Wait();
 
  private:
+  // RenderFrameObserver implementation.
   void DidFinishLoad() override;
+  void OnDestruct() override;
 
   base::RunLoop run_loop_;