Standardize usage of virtual/override/final in cc/

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 638064740b103caca69de0d10f38ddcf838101bd..ddb315f389390afd713f2387facbcf27e083e4c9 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -176,12 +176,12 @@ class LayerTreeHostImplTimeSourceAdapter : public TimeSourceClient {
         new LayerTreeHostImplTimeSourceAdapter(layer_tree_host_impl,
                                                time_source));
   }
-  virtual ~LayerTreeHostImplTimeSourceAdapter() {
+  ~LayerTreeHostImplTimeSourceAdapter() override {
     time_source_->SetClient(NULL);
     time_source_->SetActive(false);
   }
 
-  virtual void OnTimerTick() override {
+  void OnTimerTick() override {
     // In single threaded mode we attempt to simulate changing the current
     // thread by maintaining a fake thread id. When we switch from one
     // thread to another, we construct DebugScopedSetXXXThread objects that