Convert GN single-item list concat to use lists.

base/test/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.
 
 static_library("test_support_base") {
   external = true
   sources = [
     "expectations/expectation.cc",
     "expectations/expectation.h",
     "expectations/parser.cc",
@@ skipping 88 matching lines @@
   ]
 
   if (!is_posix) {
     sources -= [
       "scoped_locale.cc",
       "scoped_locale.h",
     ]
   }
   if (is_ios) {
     # iOS uses its own unit test launcher.
-    sources -= "launcher/unit_test_launcher.cc"
+    sources -= [ "launcher/unit_test_launcher.cc" ]
 
     # Pull in specific Mac files for iOS (which have been filtered out
     # by file name rules).
     set_sources_assignment_filter([])
     sources += "test_file_util_mac.cc"
   }
   if (toolkit_uses_gtk) {
-    configs += "//build/config/linux:gtk"
+    configs += [ "//build/config/linux:gtk" ]
   }
 }
 
 config("perf_test_config") {
   defines = [ "PERF_TEST" ]
 }
 
 static_library("test_support_perf") {
   external = true
   sources = [
     "run_all_perftests.cc",
   ]
   deps = [
     "//base",
     "//testing/gtest",
   ]
 
   direct_dependent_configs = [ ":perf_test_config" ]
 
   if (toolkit_uses_gtk) {
-    configs += "//build/config/linux:gtk"
+    configs += [ "//build/config/linux:gtk" ]
   }
 }
 
 static_library("run_all_unittests") {
   external = true
   sources = [
     "run_all_unittests.cc",
   ]
   deps = [
     ":test_support_base",
   ]
 }