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/SkRawAdapterCodec.h b/src/codec/SkRawAdapterCodec.h
index 8777180..29e817c 100644
--- a/src/codec/SkRawAdapterCodec.h
+++ b/src/codec/SkRawAdapterCodec.h
@@ -24,7 +24,7 @@
 
     explicit SkRawAdapterCodec(SkRawCodec*);
 
-    virtual ~SkRawAdapterCodec() {}
+    ~SkRawAdapterCodec() override {}
 
 protected: