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/effects/gradients/SkRadialGradient.cpp b/src/effects/gradients/SkRadialGradient.cpp
index f282bab..2caf905 100644
--- a/src/effects/gradients/SkRadialGradient.cpp
+++ b/src/effects/gradients/SkRadialGradient.cpp
@@ -249,7 +249,7 @@
         return sk_sp<GrFragmentProcessor>(new GrRadialGradient(args));
     }
 
-    virtual ~GrRadialGradient() { }
+    ~GrRadialGradient() override {}
 
     const char* name() const override { return "Radial Gradient"; }
 
@@ -273,7 +273,7 @@
 class GrRadialGradient::GLSLRadialProcessor : public GrGradientEffect::GLSLProcessor {
 public:
     GLSLRadialProcessor(const GrProcessor&) {}
-    virtual ~GLSLRadialProcessor() { }
+    ~GLSLRadialProcessor() override {}
 
     virtual void emitCode(EmitArgs&) override;