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/image/SkSurface_Gpu.h b/src/image/SkSurface_Gpu.h
index 5b92eeb..cc8b87d 100644
--- a/src/image/SkSurface_Gpu.h
+++ b/src/image/SkSurface_Gpu.h
@@ -17,7 +17,7 @@
 class SkSurface_Gpu : public SkSurface_Base {
 public:
     SkSurface_Gpu(sk_sp<SkGpuDevice>);
-    virtual ~SkSurface_Gpu();
+    ~SkSurface_Gpu() override;
 
     GrBackendObject onGetTextureHandle(BackendHandleAccess) override;
     bool onGetRenderTargetHandle(GrBackendObject*, BackendHandleAccess) override;