Mark overridden destructors with 'override' and remove 'virtual'

This silences a new warning in clang 5.0

Change-Id: Ieb5b75a6ffed60107c3fd16075d2ecfd515b55e8
Reviewed-on: https://skia-review.googlesource.com/10006
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/samplecode/OverView.cpp b/samplecode/OverView.cpp
index 0cd129e..62e8c2a 100644
--- a/samplecode/OverView.cpp
+++ b/samplecode/OverView.cpp
@@ -45,7 +45,7 @@
 class OverView : public SkView {
 public:
     OverView(int count, const SkViewFactory* factories[]);
-    virtual ~OverView();
+    ~OverView() override;
 
 protected:
     bool onEvent(const SkEvent&) override;