Fix some TODO(GYP) in the GN build.

third_party/WebKit/Source/BUILD.gn

 # Copyright 2014 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.
 
 import("config.gni")
 if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
 
 visibility = [ "//third_party/WebKit/*" ]
@@ skipping 63 matching lines @@
       ]
     }
 
     cflags += [
       "-Xclang",
       "-add-plugin",
       "-Xclang",
       "blink-gc-plugin",
     ]
   }
-
-  ## TODO(GYP) : gn does not yet support use_system_icu.

[brettw, 2016/05/23 22:06:26]

use_system_icu isn't required, and if we add it it should be integrated
differently than this.

-  #if (use_system_icu) {
-  #  defines += [ "USING_SYSTEM_ICU" ]
-  #}
 }
 
 # The follow configs apply to all targets except for unit tests, which rely on
 # static initializers.
 config("non_test_config") {
   cflags = []
 
   if (is_clang) {
     cflags += [ "-Wglobal-constructors" ]
   }
 }
 
 if (is_mac) {
   # This sets up precompiled headers for Mac.
   config("mac_precompiled_headers") {
     precompiled_header = rebase_path("build/mac/Prefix.h", root_build_dir)
     precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
   }
 }