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/gm/giantbitmap.cpp b/gm/giantbitmap.cpp
index e1ba737..37b76a6 100644
--- a/gm/giantbitmap.cpp
+++ b/gm/giantbitmap.cpp
@@ -66,7 +66,7 @@
         fDoRotate = doRotate;
     }
 
-    virtual ~GiantBitmapGM() { delete fBM; }
+    ~GiantBitmapGM() override { delete fBM; }
 
 protected: