Add missing .h files to .gn files that have a .cc file next to it in the gn file already.

extensions/browser/api/document_scan/BUILD.gn

 # Copyright 2016 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.
 
 source_set("document_scan") {
   sources = [
     "document_scan_api.cc",
     "document_scan_api.h",
     "document_scan_interface.cc",
     "document_scan_interface.h",
     "document_scan_interface_chromeos.cc",
+    "document_scan_interface_chromeos.h",
   ]
 
   if (!is_chromeos) {
     sources += [ "document_scan_interface_nonchromeos.cc" ]
   }
 
   deps = [
     "//extensions/common/api",
   ]
 }