gn: Remove setting |use_default_render_theme|.

Source/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")
 import("//third_party/WebKit/public/features.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 } else {
   # TODO(brettw) remove this once && early-out is checked in.
   arm_version = 0
 }
 
 declare_args() {
   # Set to true to enable the clang plugin that checks the usage of the Blink
   # garbage-collection infrastructure during compilation.
   blink_gc_plugin = false
   blink_gc_plugin_dump_graph = false
 
   blink_asserts_always_on = false
 }
 
-use_default_render_theme = use_aura
-
 # Whether Android ARM or x86 build uses OpenMAX DL FFT. Currently only
 # supported on Android ARMv7+, ia32 or x64 without webview. When enabled, this
 # will also enable WebAudio support on Android ARM, ia32 and x64. Default is
 # enabled. Whether WebAudio is actually available depends on runtime settings
 # and flags.
 use_openmax_dl_fft = is_android &&
   (current_cpu == "x86" ||
    current_cpu == "x64" ||
    (current_cpu == "arm" && arm_version >= 7) ||
    current_cpu == "mipsel")
@@ skipping 52 matching lines @@
 }
 
 # 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")