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/ops/GrMSAAPathRenderer.cpp b/src/gpu/ops/GrMSAAPathRenderer.cpp
index 04b0b14..ccbd0e2 100644
--- a/src/gpu/ops/GrMSAAPathRenderer.cpp
+++ b/src/gpu/ops/GrMSAAPathRenderer.cpp
@@ -116,7 +116,7 @@
         return new MSAAQuadProcessor(viewMatrix);
     }
 
-    virtual ~MSAAQuadProcessor() {}
+    ~MSAAQuadProcessor() override {}
 
     const char* name() const override { return "MSAAQuadProcessor"; }