Fix ENABLE_SWIFTSHADER flag.

ui/gl/BUILD.gn

Index: ui/gl/BUILD.gn
diff --git a/ui/gl/BUILD.gn b/ui/gl/BUILD.gn
index 8f9896b54cde15ba62750257c21b0a3fe57ec56d..32027848dc137d56b5abf2a231f5baeb1dfc04f4 100644
--- a/ui/gl/BUILD.gn
+++ b/ui/gl/BUILD.gn
@@ -2,10 +2,16 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/buildflag_header.gni")
+import("//build/config/chrome_build.gni")
 import("//build/config/linux/pkg_config.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
+declare_args() {
+  enable_swiftshader = false
+}
+
 use_egl = is_win || is_android || is_linux
 use_glx = use_x11
 
@@ -14,6 +20,11 @@ if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
+buildflag_header("gl_features") {
+  header = "gl_features.h"
+  flags = [ "ENABLE_SWIFTSHADER=$enable_swiftshader" ]
+}
+
 config("gl_config") {
   defines = []
   if (use_glx) {