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/codec/SkPngCodec.h b/src/codec/SkPngCodec.h
index 7a2ebae..09231f1 100644
--- a/src/codec/SkPngCodec.h
+++ b/src/codec/SkPngCodec.h
@@ -32,7 +32,7 @@
     // Assume IsPng was called and returned true.
     static SkCodec* NewFromStream(SkStream*, SkPngChunkReader* = NULL);
 
-    virtual ~SkPngCodec();
+    ~SkPngCodec() override;
 
 protected:
     // We hold the png_ptr and info_ptr as voidp to avoid having to include png.h