Standardize usage of virtual/override/final in mojo/

mojo/public/cpp/environment/lib/default_async_waiter.cc

Index: mojo/public/cpp/environment/lib/default_async_waiter.cc
diff --git a/mojo/public/cpp/environment/lib/default_async_waiter.cc b/mojo/public/cpp/environment/lib/default_async_waiter.cc
index 9211c0f75c9edeb0bb1616180b0c93b6bfa3aa06..4a588a991f4b355214a8681e19eaf3edecda8ebf 100644
--- a/mojo/public/cpp/environment/lib/default_async_waiter.cc
+++ b/mojo/public/cpp/environment/lib/default_async_waiter.cc
@@ -25,14 +25,14 @@ class RunLoopHandlerImpl : public RunLoopHandler {
                      void* closure)
       : handle_(handle), callback_(callback), closure_(closure) {}
 
-  virtual ~RunLoopHandlerImpl() { RunLoop::current()->RemoveHandler(handle_); }
+  ~RunLoopHandlerImpl() override { RunLoop::current()->RemoveHandler(handle_); }
 
   // RunLoopHandler:
-  virtual void OnHandleReady(const Handle& handle) override {
+  void OnHandleReady(const Handle& handle) override {
     NotifyCallback(MOJO_RESULT_OK);
   }
 
-  virtual void OnHandleError(const Handle& handle, MojoResult result) override {
+  void OnHandleError(const Handle& handle, MojoResult result) override {
     NotifyCallback(result);
   }