views: Mark single-argument constructors as explicit.

ui/views/widget/native_widget_unittest.cc

Index: ui/views/widget/native_widget_unittest.cc
diff --git a/ui/views/widget/native_widget_unittest.cc b/ui/views/widget/native_widget_unittest.cc
index 808649abf8720e7608a7b14a37de362a29af3690..1e2d197e9300e1c8ab02aa99f23c46ab01ffaa49 100644
--- a/ui/views/widget/native_widget_unittest.cc
+++ b/ui/views/widget/native_widget_unittest.cc
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -14,7 +14,7 @@ namespace views {
 
 class ScopedTestWidget {
  public:
-  ScopedTestWidget(internal::NativeWidgetPrivate* native_widget)
+  explicit ScopedTestWidget(internal::NativeWidgetPrivate* native_widget)
       : native_widget_(native_widget) {
   }
   ~ScopedTestWidget() {