Add compile flag for enabling the compostior

sky/engine/config.gni

 # 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("//build/config/ui.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 } else {
   # TODO(brettw) remove this once && early-out is checked in.
   is_android_webview_build = false
 }
 if (cpu_arch == "arm") {
   import("//build/config/arm.gni")
 } else {
   # TODO(brettw) remove this once && early-out is checked in.
   arm_version = 0
 }
 
 declare_args() {
   # Enables the Oilpan garbage-collection infrastructure.
   sky_enable_oilpan = false
 
   # Set to true to enable the clang plugin that checks the usage of the Blink
   # garbage-collection infrastructure during compilation.
   sky_gc_plugin = false
   sky_gc_plugin_dump_graph = false
 
   sky_asserts_always_on = false
+
+  sky_enable_compostior = false
 }
 
 # feature_defines_list ---------------------------------------------------------
 
 feature_defines_list = []
 
 if (is_debug) {
   # TODO(GYP) this should also be set when release_valgrind_build is set,
   # but we don't have that flag in the GN build yet.
   feature_defines_list += [ "WTF_USE_DYNAMIC_ANNOTATIONS=1" ]
 }
 
 if (!is_mac) {
   # Mac OS X has not implemented support for ENABLE(OPENTYPE_VERTICAL) yet.
   feature_defines_list += [ "ENABLE_OPENTYPE_VERTICAL=1" ]
 }
 
 if (sky_enable_oilpan) {
   feature_defines_list += [ "ENABLE_OILPAN=1" ]
 }
 if (sky_asserts_always_on) {
   feature_defines_list += [ "ENABLE_ASSERT=1" ]
 }
+if (sky_enable_compostior) {
+  feature_defines_list += [ "ENABLE_COMPOSITOR=1" ]
+}
 
 # feature_defines_string -------------------------------------------------------
 
 # Convert the list to a space-separated string for passing to scripts.
 # This would be the equivalent of passing '<(feature_defines)' in GYP.
 feature_defines_string = exec_script(
     "build/gn_list_to_space_separated_string.py",
     feature_defines_list,
     "trim string")