Standardize usage of virtual/override/final in sync/

sync/test/trackable_mock_invalidation.h

Index: sync/test/trackable_mock_invalidation.h
diff --git a/sync/test/trackable_mock_invalidation.h b/sync/test/trackable_mock_invalidation.h
index 56878661dff706405d52553e54d6c492ff776a40..758ff080da786baf017063b4b4302f4945a37a28 100644
--- a/sync/test/trackable_mock_invalidation.h
+++ b/sync/test/trackable_mock_invalidation.h
@@ -22,14 +22,14 @@ class TrackableMockInvalidation : public MockInvalidation {
                             const std::string& payload,
                             MockInvalidationTracker* tracker,
                             int tracking_id);
-  virtual ~TrackableMockInvalidation();
+  ~TrackableMockInvalidation() override;
 
   // Forwards notice of the acknowledgement of this invalidation to the
   // |tracker_|.
-  virtual void Acknowledge() override;
+  void Acknowledge() override;
 
   // Forwards notice of the drop of this invalidation to the |tracker_|.
-  virtual void Drop() override;
+  void Drop() override;
 
   // Returns the integer used to identify this object with the |tracker_|.
   int GetTrackingId();