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/SkTableColorFilter.cpp b/src/effects/SkTableColorFilter.cpp
index 393543b..f09d5ed 100644
--- a/src/effects/SkTableColorFilter.cpp
+++ b/src/effects/SkTableColorFilter.cpp
@@ -80,7 +80,7 @@
         }
     }
 
-    virtual ~SkTable_ColorFilter() { delete fBitmap; }
+    ~SkTable_ColorFilter() override { delete fBitmap; }
 
     bool asComponentTable(SkBitmap* table) const override;
     sk_sp<SkColorFilter> makeComposed(sk_sp<SkColorFilter> inner) const override;