Standardize usage of virtual/override/final specifiers in sync/.

sync/syncable/parent_child_index_unittest.cc

Index: sync/syncable/parent_child_index_unittest.cc
diff --git a/sync/syncable/parent_child_index_unittest.cc b/sync/syncable/parent_child_index_unittest.cc
index 5ae9d27bd87faae8b337265a1fe5c25d53b7534d..c0c9e82406cc46a980880947e42a9ba1c78717a1 100644
--- a/sync/syncable/parent_child_index_unittest.cc
+++ b/sync/syncable/parent_child_index_unittest.cc
@@ -21,7 +21,7 @@ static const std::string kCacheGuid = "8HhNIHlEOCGQbIAALr9QEg==";
 
 class ParentChildIndexTest : public testing::Test {
  public:
-  virtual void TearDown() {
+  void TearDown() override {
     // To make memory management easier, we take ownership of all EntryKernels
     // returned by our factory methods and delete them here.
     STLDeleteElements(&owned_entry_kernels_);