[GN]: BUILD file housecleaning

build/config/BUILD.gn

 # Copyright (c) 2013 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/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
@@ skipping 431 matching lines @@
     # error is generated only in C mode for ANSI compatibility. It conflicts
     # with precompiled headers since the source file that's "compiled" for
     # making the precompiled header is empty.
     #
     # This error doesn't happen every time. In VS2013, it seems if the .pch
     # file doesn't exist, no error will be generated (probably MS tested this
     # case but forgot the other one?). To reproduce this error, do a build,
     # then delete the precompile.c.obj file, then build again.
     cflags_c = [ "/wd4206" ]
   } else if (is_mac && !is_official_build && !use_goma) {
-    # TODO(andybons): enable this when GCC PCH support in the binary has been
-    # rolled.
-    #precompiled_header = "build/precompile.h"
-    #precompiled_source = "//build/precompile.h"
+    precompiled_header = "build/precompile.h"
+    precompiled_source = "//build/precompile.h"
   }
 }