Disable tests failing on official builders

chrome/test/functional/PYAUTO_TESTS

 # Copyright (c) 2010 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.
 #
 # This file lists the pyauto tests that run as a part of the functional test
 # suite.
 #
 # Tests can be enabled on a per-platform basis. Tests not listed here will
 # not be run.
 #
@@ skipping 28 matching lines @@
     'prefs',
     'special_tabs',
     'test_basic.SimpleTest.testCanOpenGoogle',
     'themes',
     'translate',
   ],
 
   'win': [
     # testBookmarkBarVisible fails on windows. crbug.com/42823
     '-bookmark_bar.BookmarkBarTest.testBookmarkBarVisible',
+    # download test fails on official builder, thru buildbot only.
+    # crbug.com/50481
+    '-downloads.DownloadsTest.testZip',
   ],
 
   'mac': [
     # Keychain popups make password & autofill tests difficult. crbug.com/49378
     '-passwords',
     '-autofill',
+    # codesign tests should run *after* signing. crbug.com/50481
+    '-codesign',
   ],
 
   'linux': [  # linux != chromeos
     '-omnibox',  # http://crbug.com/44203
     '-browser.BrowserTest.testWindowResize',  # crbug.com/44963
     '-content.ContentTest.testThreeWindows',  # crbug.com/47457
   ],
 
   # ChromeOS is linux, but note that this section does not include the
   # entries in the linux section above.
   'chromeos': [
     # you cannot resize browser window on chromeos
     '-browser.BrowserTest.testWindowResize',
   ],
 }