Switch to LayoutThemeMobile when emulating mobile device in DevTools.

third_party/WebKit/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")
 }
@@ skipping 49 matching lines @@
 
 if (is_android) {
   feature_defines_list += [ "WTF_USE_LOW_QUALITY_IMAGE_INTERPOLATION=1" ]
 } else {
   feature_defines_list += [
     "WTF_USE_ICCJPEG=1",
     "WTF_USE_QCMSLIB=1",
   ]
 }
 
-if (is_mac) {
-  feature_defines_list += [ "WTF_USE_NEW_THEME=1" ]
-}
-
 if (use_webaudio_ffmpeg) {
   feature_defines_list += [ "WTF_USE_WEBAUDIO_FFMPEG=1" ]
 }
 
 if (use_openmax_dl_fft) {
   feature_defines_list += [ "WTF_USE_WEBAUDIO_OPENMAX_DL_FFT=1" ]
 }
 
 if (use_default_render_theme) {
   feature_defines_list += [ "WTF_USE_DEFAULT_RENDER_THEME=1" ]
 }
 if (enable_oilpan) {
   feature_defines_list += [ "ENABLE_OILPAN=1" ]
 }
 if (blink_asserts_off_in_release && !is_debug) {
   feature_defines_list += [ "ENABLE_ASSERT=0" ]
 }
 
 # 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")