Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

scripts/slave/recipe_modules/chromium/api.py

 # Copyright 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 re
 
 from recipe_engine import recipe_api
 from recipe_engine import util as recipe_util
 
 class TestLauncherFilterFileInputPlaceholder(recipe_util.InputPlaceholder):
@@ skipping 562 matching lines @@
       data = '\n'.join(sorted_isolated_targets) + '\n'
       args += ['--swarming-targets-file', self.m.raw_io.input(data)]
 
     if android_version_code:
       args += ['--android-version-code=%s' % android_version_code]
     if android_version_name:
       args += ['--android-version-name=%s' % android_version_name]
 
     args += [build_dir]
 
-    # This runs with no env being passed along, so we get a clean environment
-    # without any GYP_DEFINES being present to cause confusion.
+    # This runs with an almost-bare env being passed along, so we get a clean
+    # environment without any GYP_DEFINES being present to cause confusion.
     kwargs = {
       'name': name or 'generate_build_files',
       'script': self.m.path['checkout'].join('tools', 'mb', 'mb.py'),
       'args': args,
+      'env': {
+        'GOMA_SERVICE_ACCOUNT_JSON_FILE': self.m.goma.service_account_json_path,
+      }
     }
     if self.c.TARGET_CROS_BOARD:
       # Wrap 'runhooks' through 'cros chrome-sdk'
       kwargs['wrapper'] = self.get_cros_chrome_sdk_wrapper(clean=True)
 
     self.m.python(**kwargs)
 
     # Comes after self.m.python so the log appears in the correct step result.
     result = self.m.step.active_result
     if isolated_targets and result:
@@ skipping 118 matching lines @@
       }))
 
   def get_annotate_by_test_name(self, test_name):
     return 'graphing'
 
   def download_lto_plugin(self):
     return self.m.python(
         name='download LTO plugin',
         script=self.m.path['checkout'].join(
             'build', 'download_gold_plugin.py'))