Update Native Client Authors to be Chromium Authors for the code moved from the

ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_pdf.cc

-// Copyright (c) 2011 The Native Client Authors. All rights reserved.
+// Copyright (c) 2011 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 "native_client/src/shared/ppapi_proxy/plugin_ppb_pdf.h"
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
 
@@ skipping 234 matching lines @@
     HistogramPDFPageCount,
     UserMetricsRecordAction,
     HasUnsupportedFeature,
     SaveAs
   };
   return &pdf_interface;
 }
 
 }  // namespace ppapi_proxy