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/tests/CachedDecodingPixelRefTest.cpp b/tests/CachedDecodingPixelRefTest.cpp
index d9a9e9e..c423441 100644
--- a/tests/CachedDecodingPixelRefTest.cpp
+++ b/tests/CachedDecodingPixelRefTest.cpp
@@ -37,7 +37,7 @@
     : INHERITED(GetMyInfo(colorType)), fType(type), fReporter(reporter) {
         SkASSERT((fType <= kLast_TestType) && (fType >= 0));
     }
-    virtual ~TestImageGenerator() { }
+    ~TestImageGenerator() override {}
 
 protected:
     static SkImageInfo GetMyInfo(SkColorType colorType) {