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/ResourceCacheTest.cpp b/tests/ResourceCacheTest.cpp
index 8051191..9573161 100644
--- a/tests/ResourceCacheTest.cpp
+++ b/tests/ResourceCacheTest.cpp
@@ -295,7 +295,7 @@
         return new TestResource(gpu, size);
     }
 
-    ~TestResource() {
+    ~TestResource() override {
         --fNumAlive;
         SkSafeUnref(fToDelete);
     }