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/gpu/effects/GrPorterDuffXferProcessor.cpp b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
index f984355..3dc5325 100644
--- a/src/gpu/effects/GrPorterDuffXferProcessor.cpp
+++ b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
@@ -589,7 +589,7 @@
 public:
     GLPDLCDXferProcessor(const GrProcessor&) : fLastAlpha(SK_MaxU32) {}
 
-    virtual ~GLPDLCDXferProcessor() {}
+    ~GLPDLCDXferProcessor() override {}
 
     static void GenKey(const GrProcessor& processor, const GrShaderCaps& caps,
                        GrProcessorKeyBuilder* b) {}