Remove ui_compositor_image_transport, it has no effect.

build/config/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.
 
 import("//build/config/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
@@ skipping 47 matching lines @@
 config("feature_flags") {
   # Don't use deprecated V8 APIs anywhere.
   defines = [ "V8_DEPRECATION_WARNINGS" ]
   if (dcheck_always_on) {
     defines += [ "DCHECK_ALWAYS_ON=1" ]
   }
   if (use_udev) {
     # TODO(brettw) should probably be "=1".
     defines += [ "USE_UDEV" ]
   }
-  if (ui_compositor_image_transport) {
-    # TODO(brettw) should probably be "=1".
-    defines += [ "UI_COMPOSITOR_IMAGE_TRANSPORT" ]
-  }
   if (use_ash) {
     defines += [ "USE_ASH=1" ]
   }
   if (use_aura) {
     defines += [ "USE_AURA=1" ]
   }
   if (use_pango) {
     defines += [ "USE_PANGO=1" ]
   }
   if (use_cairo) {
@@ skipping 308 matching lines @@
       #
       # TODO(sof): determine VS2015 status and retire the setting from all
       # precompiled configurations.
       cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_header = "build/precompile.h"
       precompiled_source = "//build/precompile.h"
     }
   }
 }