Style cleanup in preparation for auto-linting base/.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/552004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@37164 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/values_unittest.cc b/base/values_unittest.cc
index a20f3c8c..a0ab47f 100644
--- a/base/values_unittest.cc
+++ b/base/values_unittest.cc
@@ -169,7 +169,7 @@
 // properly deleted by modifying the value of external flag on destruction.
 class DeletionTestValue : public Value {
  public:
-  DeletionTestValue(bool* deletion_flag) : Value(TYPE_NULL) {
+  explicit DeletionTestValue(bool* deletion_flag) : Value(TYPE_NULL) {
     Init(deletion_flag);  // Separate function so that we can use ASSERT_*
   }