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/include/effects/SkLayerDrawLooper.h b/include/effects/SkLayerDrawLooper.h
index c0a9534..3a35140 100644
--- a/include/effects/SkLayerDrawLooper.h
+++ b/include/effects/SkLayerDrawLooper.h
@@ -15,7 +15,7 @@
 
 class SK_API SkLayerDrawLooper : public SkDrawLooper {
 public:
-    virtual ~SkLayerDrawLooper();
+    ~SkLayerDrawLooper() override;
 
     /**
      *  Bits specifies which aspects of the layer's paint should replace the