chore: fix chromium-style errors in frame_subscriber

The chromium-style linter we run in the GN build was throwing errors
about these issues.

atom/browser/api/frame_subscriber.h

@@ -28,7 +28,7 @@ class FrameSubscriber : public content::WebContentsObserver {
                   content::WebContents* web_contents,
                   const FrameCaptureCallback& callback,
                   bool only_dirty);
-  ~FrameSubscriber();
+  ~FrameSubscriber() override;
 
  private:
   gfx::Rect GetDamageRect();