Add //build/config/BUILD.gn to exec whitelist for GN.

BUILD.gn

 # Copyright 2014 The LibYuv Project Authors. All rights reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS. All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/arm.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 config("libyuv_config") {
   include_dirs = [
     ".",
     "include",
   ]
 }
 
 use_neon = current_cpu == "arm64" || (current_cpu == "arm" && (arm_use_neon || arm_optionally_use_neon))
 
 source_set("libyuv") {
   sources = [
+    # Headers
     "include/libyuv.h",
     "include/libyuv/basic_types.h",
     "include/libyuv/compare.h",
     "include/libyuv/convert.h",
     "include/libyuv/convert_argb.h",
     "include/libyuv/convert_from.h",
     "include/libyuv/convert_from_argb.h",
     "include/libyuv/cpu_id.h",
     "include/libyuv/mjpeg_decoder.h",
     "include/libyuv/planar_functions.h",
     "include/libyuv/rotate.h",
     "include/libyuv/rotate_argb.h",
     "include/libyuv/rotate_row.h",
     "include/libyuv/row.h",
     "include/libyuv/scale.h",
     "include/libyuv/scale_argb.h",
     "include/libyuv/scale_row.h",
     "include/libyuv/version.h",
     "include/libyuv/video_common.h",
 
-    # sources.
+    # Source Files
     "source/compare.cc",
     "source/compare_common.cc",
     "source/compare_gcc.cc",
     "source/compare_win.cc",
     "source/convert.cc",
     "source/convert_argb.cc",
     "source/convert_from.cc",
     "source/convert_from_argb.cc",
     "source/convert_jpeg.cc",
     "source/convert_to_argb.cc",
@@ skipping 52 matching lines @@
   if (is_nacl) {
     # Always enable optimization under NaCl to workaround crbug.com/538243 .
     configs -= [ "//build/config/compiler:default_optimization" ]
     configs += [ "//build/config/compiler:optimize_max" ]
   }
 }
 
 if (use_neon) {
   static_library("libyuv_neon") {
     sources = [
+      # ARM Source Files
       "source/compare_neon.cc",
       "source/compare_neon64.cc",
       "source/rotate_neon.cc",
       "source/rotate_neon64.cc",
       "source/row_neon.cc",
       "source/row_neon64.cc",
       "source/scale_neon.cc",
       "source/scale_neon64.cc",
     ]
 
     public_configs = [ ":libyuv_config" ]
 
     if (current_cpu != "arm64") {
       configs -= [ "//build/config/compiler:compiler_arm_fpu" ]
       cflags = [ "-mfpu=neon" ]
     }
   }
 }