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/gm/multipicturedraw.cpp b/gm/multipicturedraw.cpp
index 35f4c12..e7d4baa 100644
--- a/gm/multipicturedraw.cpp
+++ b/gm/multipicturedraw.cpp
@@ -470,7 +470,7 @@
             }
         }
 
-        virtual ~MultiPictureDraw() {
+        ~MultiPictureDraw() override {
             for (int i = 0; i < kNumPictures; ++i) {
                 SkSafeUnref(fPictures[i]);
             }