Define a variable to distinguish system_icu from bundled_icu in Blink

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 49 matching lines @@
     # timesNewRoman.unstatic.3258 and colorTransparent.unstatic.4879.
     cflags += [ "-Wno-uninitialized" ]
   }
 
   if (is_clang) {
     if (blink_gc_plugin && clang_use_chrome_plugins) {
       # TODO(GYP)
       #'cflags': ['<!@(../../../tools/clang/scripts/blink_gc_plugin_flags.py enable-oilpan=<(enable_oilpan) dump-graph=<(blink_gc_plugin_dump_graph))'],
     }
   }
+
+  ## TODO(GYP) : gn does not yet support use_system_icu.
+  #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" ]
   }
 }