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/SkCoreBlitters.h b/src/core/SkCoreBlitters.h
index 85392dd..0e8b819 100644
--- a/src/core/SkCoreBlitters.h
+++ b/src/core/SkCoreBlitters.h
@@ -85,7 +85,7 @@
 public:
     SkA8_Shader_Blitter(const SkPixmap& device, const SkPaint& paint,
                         SkShader::Context* shaderContext);
-    virtual ~SkA8_Shader_Blitter();
+    ~SkA8_Shader_Blitter() override;
     void blitH(int x, int y, int width) override;
     void blitAntiH(int x, int y, const SkAlpha antialias[], const int16_t runs[]) override;
     void blitMask(const SkMask&, const SkIRect&) override;
@@ -156,7 +156,7 @@
 public:
     SkARGB32_Shader_Blitter(const SkPixmap& device, const SkPaint& paint,
                             SkShader::Context* shaderContext);
-    virtual ~SkARGB32_Shader_Blitter();
+    ~SkARGB32_Shader_Blitter() override;
     void blitH(int x, int y, int width) override;
     void blitV(int x, int y, int height, SkAlpha alpha) override;
     void blitRect(int x, int y, int width, int height) override;