Remove some global UI defines

third_party/WebKit/Source/config.gni

Index: third_party/WebKit/Source/config.gni
diff --git a/third_party/WebKit/Source/config.gni b/third_party/WebKit/Source/config.gni
index b291c35923597f156560dcf7ca161fdcc2704568..8aab3e02bbcbf2fcde0936b2489e57c921a72996 100644
--- a/third_party/WebKit/Source/config.gni
+++ b/third_party/WebKit/Source/config.gni
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
+import("//third_party/WebKit/public/public_features.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
@@ -67,6 +68,9 @@ if (use_openmax_dl_fft) {
 }
 
 if (use_default_render_theme) {
+  # Mirrors the USE_DEFAULT_RENDER_THEME buildflag_header in WebKit/public.
+  # If/when Blink can use buildflag headers, this should be removed in
+  # preference to that.
   feature_defines_list += [ "WTF_USE_DEFAULT_RENDER_THEME=1" ]
 }
 if (blink_logging_always_on) {