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/src/gpu/effects/GrConvexPolyEffect.h b/src/gpu/effects/GrConvexPolyEffect.h
index 89a8124..34f8b22 100644
--- a/src/gpu/effects/GrConvexPolyEffect.h
+++ b/src/gpu/effects/GrConvexPolyEffect.h
@@ -57,7 +57,7 @@
      */
     static sk_sp<GrFragmentProcessor> Make(GrPrimitiveEdgeType, const SkRect&);
 
-    virtual ~GrConvexPolyEffect();
+    ~GrConvexPolyEffect() override;
 
     const char* name() const override { return "ConvexPoly"; }