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/core/SkResourceCache.cpp b/src/core/SkResourceCache.cpp
index e7c2f94..9f5bc25 100644
--- a/src/core/SkResourceCache.cpp
+++ b/src/core/SkResourceCache.cpp
@@ -91,7 +91,7 @@
     // The pixelref will ref() the colortable (if not NULL), and unref() in destructor
     SkOneShotDiscardablePixelRef(const SkImageInfo&, SkDiscardableMemory*, size_t rowBytes,
                                  SkColorTable*);
-    ~SkOneShotDiscardablePixelRef();
+    ~SkOneShotDiscardablePixelRef() override;
 
 protected:
     bool onNewLockPixels(LockRec*) override;