Cleanup coding style

extensions/common/stack_frame_unittest.cc

 // Copyright 2013 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.
 
+#include <string>
+
 #include "extensions/common/stack_frame.h"
 
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string16.h"
 #include "base/strings/utf_string_conversions.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 using base::UTF8ToUTF16;
 
@@ skipping 61 matching lines @@
       "function_name (https://www.url.com/foo.html:100:201a)");
   // Negative column number.
   AssertStackFrameInvalid(
       "function_name (https://www.url.com/foo.html:100:-201)");
   // Extra trailing ')'
   AssertStackFrameInvalid(
       "function_name (https://www.url.com/foo.html:100:201))");
 }
 
 }  // namespace extensions