Standardize usage of virtual/override/final in mojo/

mojo/public/cpp/application/lib/application_impl.cc

Index: mojo/public/cpp/application/lib/application_impl.cc
diff --git a/mojo/public/cpp/application/lib/application_impl.cc b/mojo/public/cpp/application/lib/application_impl.cc
index 6a5560dd32b7cc0d10f5c1ab978159f1a6e68297..30b04e9368e536716338f9d6b558e2a680fa4804 100644
--- a/mojo/public/cpp/application/lib/application_impl.cc
+++ b/mojo/public/cpp/application/lib/application_impl.cc
@@ -15,9 +15,9 @@ class ApplicationImpl::ShellPtrWatcher : public ErrorHandler {
  public:
   ShellPtrWatcher(ApplicationImpl* impl) : impl_(impl) {}
 
-  virtual ~ShellPtrWatcher() {}
+  ~ShellPtrWatcher() override {}
 
-  virtual void OnConnectionError() override { impl_->OnShellError(); }
+  void OnConnectionError() override { impl_->OnShellError(); }
 
  private:
   ApplicationImpl* impl_;