views: Mark single-argument constructors as explicit.

ui/views/controls/scrollbar/scroll_bar.h

Index: ui/views/controls/scrollbar/scroll_bar.h
diff --git a/ui/views/controls/scrollbar/scroll_bar.h b/ui/views/controls/scrollbar/scroll_bar.h
index 1ed3d5c5e230c4cbd1ca51c3a322e1308d19a6c3..52bbb177ea793499268606e66bc10cbc0372412c 100644
--- a/ui/views/controls/scrollbar/scroll_bar.h
+++ b/ui/views/controls/scrollbar/scroll_bar.h
@@ -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.
 
@@ -92,7 +92,7 @@ class VIEWS_EXPORT ScrollBar : public View {
   // Create new scrollbar, either horizontal or vertical. These are protected
   // since you need to be creating either a NativeScrollBar or a
   // BitmapScrollBar.
-  ScrollBar(bool is_horiz);
+  explicit ScrollBar(bool is_horiz);
 
  private:
   const bool is_horiz_;