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/core/SkBitmapCache.cpp b/src/core/SkBitmapCache.cpp
index 0751089..76b4588 100644
--- a/src/core/SkBitmapCache.cpp
+++ b/src/core/SkBitmapCache.cpp
@@ -228,7 +228,7 @@
         fMipMap->attachToCacheAndRef();
     }
 
-    virtual ~MipMapRec() {
+    ~MipMapRec() override {
         fMipMap->detachFromCacheAndUnref();
     }