update_engine: Add override when possible.

Google Style Guide requires to include the "override" keyword
when overriding a method on a derived class, so the compiler will
catch errors if the method is not overriding a member of the base
class.

This patch introduces the "override" keyword when possible.

BUG=None
TEST=FEATURES=test emerge-link update_engine

Change-Id: Ie83d115c5730f3b35b3d95859a54bc1a48e0be7b
Reviewed-on: https://chromium-review.googlesource.com/228928
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/chrome_browser_proxy_resolver.h b/chrome_browser_proxy_resolver.h
index 80b1b2a..8b47672 100644
--- a/chrome_browser_proxy_resolver.h
+++ b/chrome_browser_proxy_resolver.h
@@ -30,12 +30,12 @@
 class ChromeBrowserProxyResolver : public ProxyResolver {
  public:
   explicit ChromeBrowserProxyResolver(DBusWrapperInterface* dbus);
-  virtual ~ChromeBrowserProxyResolver();
+  ~ChromeBrowserProxyResolver() override;
   bool Init();
 
-  virtual bool GetProxiesForUrl(const std::string& url,
-                                ProxiesResolvedFn callback,
-                                void* data);
+  bool GetProxiesForUrl(const std::string& url,
+                        ProxiesResolvedFn callback,
+                        void* data) override;
   void set_timeout(int seconds) { timeout_ = seconds; }
 
   // Public for test