Convert GN single-item list concat to use lists.

tools/gn/secondary/third_party/libxml/BUILD.gn

 # Copyright (c) 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.
 
 # Define an "os_include" variable that points at the OS-specific generated
 # headers.  These were generated by running the configure script offline.
 if (is_linux) {
   os_include = "linux"
 } else if (is_mac || is_ios) {
   os_include = "mac"
@@ skipping 122 matching lines @@
     "src/xmlschemastypes.c",
     "src/xmlstring.c",
     "src/xmlunicode.c",
     "src/xmlwriter.c",
     "src/xpath.c",
     "src/xpointer.c",
     "win32/config.h",
     "win32/include/libxml/xmlversion.h",
   ]
 
-  configs -= "//build/config/compiler:chromium_code"
-  configs += "//build/config/compiler:no_chromium_code"
+  configs -= [ "//build/config/compiler:chromium_code" ]
+  configs += [ "//build/config/compiler:no_chromium_code" ]
 
   direct_dependent_configs = [ ":libxml_config" ]
   forward_dependent_configs_from = [ "//third_party/icu:icuuc" ]
 
   deps = [
     "//third_party/icu:icuuc",
     "//third_party/zlib",
   ]
 
   if (is_win) {
@@ skipping 23 matching lines @@
 
       # See http://crbug.com/138571#c8
       "-Wno-ignored-attributes",
     ]
   }
 
   include_dirs = [
     "$os_include",
   ]
 }