Rework how MB and GN handle concurrent links.

tools/mb/mb_unittest.py

 #!/usr/bin/python
 # Copyright 2015 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.
 
 """Tests for mb.py."""
 
 import json
 import StringIO
 import os
@@ skipping 157 matching lines @@
     'bad_nested_config': {
       'mixins': ['chrome_with_codecs'],
     },
     'rel': {
       'gn_args': 'is_debug=false',
     },
   },
 }
 """
 
+
+GYP_HACKS_CONFIG = """\
+{
+  'masters': {
+    'chromium': {},
+    'fake_master': {
+      'fake_builder': 'fake_config',
+    },
+  },
+  'configs': {
+    'fake_config': ['fake_mixin'],
+  },
+  'mixins': {
+    'fake_mixin': {
+      'type': 'gyp',
+      'gn_args': '',
+      'gyp_defines':
+         ('foo=bar llvm_force_head_revision=1 '
+          'gyp_link_concurrency=1 baz=1'),
+    },
+  },
+}
+"""
+
+
 class UnitTest(unittest.TestCase):
   def fake_mbw(self, files=None, win32=False):
     mbw = FakeMBW(win32=win32)
     mbw.files.setdefault(mbw.default_config, TEST_CONFIG)
     mbw.files.setdefault(
         mbw.ToAbsPath('//build/args/bots/fake_master/fake_gn_args_bot.gn'),
         'is_debug = false\n')
     if files:
       for path, contents in files.items():
         mbw.files[path] = contents
@@ skipping 278 matching lines @@
 
   def test_validate(self):
     mbw = self.fake_mbw()
     self.check(['validate'], mbw=mbw, ret=0)
 
   def test_bad_validate(self):
     mbw = self.fake_mbw()
     mbw.files[mbw.default_config] = TEST_BAD_CONFIG
     self.check(['validate'], mbw=mbw, ret=1)
 
+  def test_gyp_env_hacks(self):
+    mbw = self.fake_mbw()
+    mbw.files[mbw.default_config] = GYP_HACKS_CONFIG
+    self.check(['lookup', '-c', 'fake_config'], mbw=mbw,
+               ret=0,
+               out=("GYP_DEFINES='foo=bar baz=1'\n"
+                    "GYP_LINK_CONCURRENCY=1\n"
+                    "LLVM_FORCE_HEAD_REVISION=1\n"
+                    "python build/gyp_chromium -G output_dir=_path_\n"))
+
 
 if __name__ == '__main__':
   unittest.main()