Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/shell/renderer/layout_test/blink_test_runner.h

Index: content/shell/renderer/layout_test/blink_test_runner.h
diff --git a/content/shell/renderer/layout_test/blink_test_runner.h b/content/shell/renderer/layout_test/blink_test_runner.h
index 2b1b276e2024ec815bfbda82da0460efc79fde03..ceb511478113a5c4e2b892d6b124b2e09164c079 100644
--- a/content/shell/renderer/layout_test/blink_test_runner.h
+++ b/content/shell/renderer/layout_test/blink_test_runner.h
@@ -179,6 +179,9 @@ class BlinkTestRunner : public RenderViewObserver,
   void OnReplyBluetoothManualChooserEvents(
       const std::vector<std::string>& events);
 
+  // RenderViewObserver implementation.
+  void OnDestruct() override;
+
   // After finishing the test, retrieves the audio, text, and pixel dumps from
   // the TestRunner library and sends them to the browser process.
   void CaptureDump();