chromeos: Add "virtual" to d'tor to appease clang.

The lack of a "virtual" on InitialBrowserWindowObserver's
destructor generates an error on the linux_chromeos_clang
builder.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/8586039

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110566 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/chromeos/legacy_window_manager/initial_browser_window_observer.h

@@ -29,7 +29,7 @@ namespace chromeos {
 class InitialBrowserWindowObserver : public content::NotificationObserver {
  public:
   InitialBrowserWindowObserver();
-  ~InitialBrowserWindowObserver() {}
+  virtual ~InitialBrowserWindowObserver() {}
 
   // content::NotificationObserver implementation:
   virtual void Observe(int type,