GN: expunge all environment variables that have no effect on GN

infra/bots/recipe_modules/flavor/gn_flavor.py

 # Copyright 2016 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 default_flavor
 
 """GN flavor utils, used for building Skia with GN."""
 class GNFlavorUtils(default_flavor.DefaultFlavorUtils):
   def supported(self):
     extra_config = self.m.vars.builder_cfg.get('extra_config', '')
 
     return any([
       extra_config == 'GN',
       extra_config == 'Fast',
       extra_config.startswith('SK')
     ])
 
+  def _run(self, title, cmd):
+    path = self.m.vars.default_env['PATH']
+    self.m.vars.default_env = {'PATH': path}
+    self.m.run(self.m.step, title, cmd=cmd, cwd=self.m.vars.skia_dir)
+
   def compile(self, unused_target, **kwargs):
     """Build Skia with GN."""
     compiler      = self.m.vars.builder_cfg.get('compiler',      '')
     configuration = self.m.vars.builder_cfg.get('configuration', '')
     extra_config  = self.m.vars.builder_cfg.get('extra_config',  '')
     os            = self.m.vars.builder_cfg.get('os',            '')
 
     cc, cxx = 'cc', 'c++'
     extra_cflags = []
 
@@ skipping 20 matching lines @@
 
     quote = lambda x: '"%s"' % x
     gn_args = ' '.join('%s=%s' % (k,v) for (k,v) in sorted({
         'cc': quote(cc),
         'cxx': quote(cxx),
         'compiler_prefix': quote(compiler_prefix),
         'extra_cflags': quote(' '.join(extra_cflags)),
         'is_debug': 'true' if configuration == 'Debug' else 'false',
     }.iteritems()))
 
-    run = lambda title, cmd: self.m.run(self.m.step, title, cmd=cmd,
-                                        cwd=self.m.vars.skia_dir, **kwargs)
-
-    run('fetch-gn', [self.m.vars.skia_dir.join('bin', 'fetch-gn')])
-    run('gn gen', ['gn', 'gen', self.out_dir, '--args=' + gn_args])
-    run('ninja', ['ninja', '-C', self.out_dir])
+    self._run('fetch-gn', [self.m.vars.skia_dir.join('bin', 'fetch-gn')])
+    self._run('gn gen', ['gn', 'gen', self.out_dir, '--args=' + gn_args])
+    self._run('ninja', ['ninja', '-C', self.out_dir])