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

infra/bots/recipe_modules/flavor/gn_android_flavor.py

Index: infra/bots/recipe_modules/flavor/gn_android_flavor.py
diff --git a/infra/bots/recipe_modules/flavor/gn_android_flavor.py b/infra/bots/recipe_modules/flavor/gn_android_flavor.py
index 9f3480a1a7f593012211aa128a835a1d4dad8526..12832ecc83ad88380d43dcabf034e5aedd4f2446 100644
--- a/infra/bots/recipe_modules/flavor/gn_android_flavor.py
+++ b/infra/bots/recipe_modules/flavor/gn_android_flavor.py
@@ -9,6 +9,11 @@ class GNAndroidFlavorUtils(default_flavor.DefaultFlavorUtils):
   def supported(self):
     return 'GN_Android' == self.m.vars.builder_cfg.get('extra_config', '')
 
+  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, env={})
+
   def compile(self, unused_target, **kwargs):
     compiler      = self.m.vars.builder_cfg.get('compiler')
     configuration = self.m.vars.builder_cfg.get('configuration')
@@ -35,9 +40,6 @@ class GNAndroidFlavorUtils(default_flavor.DefaultFlavorUtils):
         'target_cpu': quote(target_arch),
     }.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])