Standardize usage of virtual/override/final specifiers.

ash/shell/window_watcher.cc

Index: ash/shell/window_watcher.cc
diff --git a/ash/shell/window_watcher.cc b/ash/shell/window_watcher.cc
index f56cf78983eb7f11324f26c3e3ceeae8f495ad40..990cef1e949d1a200da5bcff02065b771875412b 100644
--- a/ash/shell/window_watcher.cc
+++ b/ash/shell/window_watcher.cc
@@ -25,14 +25,13 @@ class WindowWatcher::WorkspaceWindowWatcher : public aura::WindowObserver {
   explicit WorkspaceWindowWatcher(WindowWatcher* watcher) : watcher_(watcher) {
   }
 
-  virtual ~WorkspaceWindowWatcher() {
-  }
+  ~WorkspaceWindowWatcher() override {}
 
-  virtual void OnWindowAdded(aura::Window* new_window) override {
+  void OnWindowAdded(aura::Window* new_window) override {
     new_window->AddObserver(watcher_);
   }
 
-  virtual void OnWillRemoveWindow(aura::Window* window) override {
+  void OnWillRemoveWindow(aura::Window* window) override {
     DCHECK(window->children().empty());
     window->RemoveObserver(watcher_);
   }