Enforce marking "override" for functions overriding Blink.

media/blink/multibuffer_data_source_unittest.cc

Index: media/blink/multibuffer_data_source_unittest.cc
diff --git a/media/blink/multibuffer_data_source_unittest.cc b/media/blink/multibuffer_data_source_unittest.cc
index 661840dafc822a3110b6b0716f58a5534b4a902f..3890af9448c812c835655aa9bf41c5c4a942db81 100644
--- a/media/blink/multibuffer_data_source_unittest.cc
+++ b/media/blink/multibuffer_data_source_unittest.cc
@@ -47,7 +47,7 @@ class TestMultiBufferDataProvider : public ResourceMultiBufferDataProvider {
       : ResourceMultiBufferDataProvider(url_data, pos), loading_(false) {
     CHECK(test_data_providers.insert(this).second);
   }
-  ~TestMultiBufferDataProvider() {
+  ~TestMultiBufferDataProvider() override {
     CHECK_EQ(static_cast<size_t>(1), test_data_providers.erase(this));
   }
   void Start() override {