Standardize usage of virtual/override/final specifiers.

ui/views/controls/combobox/combobox.cc

Index: ui/views/controls/combobox/combobox.cc
diff --git a/ui/views/controls/combobox/combobox.cc b/ui/views/controls/combobox/combobox.cc
index 22689c736ede2a9a98656b25deda80fbdeb0b8ee..e1c7586f2de4cbf49502916bd876eecbb9099ab5 100644
--- a/ui/views/controls/combobox/combobox.cc
+++ b/ui/views/controls/combobox/combobox.cc
@@ -92,9 +92,9 @@ class TransparentButton : public CustomButton {
       : CustomButton(listener) {
     SetAnimationDuration(LabelButton::kHoverAnimationDurationMs);
   }
-  virtual ~TransparentButton() {}
+  ~TransparentButton() override {}
 
-  virtual bool OnMousePressed(const ui::MouseEvent& mouse_event) override {
+  bool OnMousePressed(const ui::MouseEvent& mouse_event) override {
     parent()->RequestFocus();
     return true;
   }