gn: Stop checking for gyp-style build dirs.

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/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
 import("//build/config/pch.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/toolchain/goma.gni")
 
-# One common error that happens is that GYP-generated headers within gen/ get
-# included rather than the GN-generated ones within gen/ subdirectories.
-# TODO(GYP_GONE): Remove once GYP is gone (as well as exec_script exception).
-assert(
-    exec_script("//build/dir_exists.py", [ "obj.host" ], "string") == "False",
-    "GYP artifacts detected in $root_build_dir.$0x0A" +
-        "You must wipe this directory before building with GN.")
-
 declare_args() {
   # When set (the default) enables C++ iterator debugging in debug builds.
   # Iterator debugging is always off in release builds (technically, this flag
   # affects the "debug" config, which is always available but applied by
   # default only in debug builds).
   #
   # Iterator debugging is generally useful for catching bugs. But it can
   # introduce extra locking to check the state of an iterator against the state
   # of the current object. For iterator- and thread-heavy code, this can
   # significantly slow execution.
@@ skipping 356 matching lines @@
       #
       # TODO(sof): determine VS2015 status and retire the setting from all
       # precompiled configurations.
       cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_header = "build/precompile.h"
       precompiled_source = "//build/precompile.h"
     }
   }
 }