"Root devices" step fails on Android Tests (dbg) https://build.chromium.org/p/chromium.linux/builders/Android%20Tests%20(dbg) 

Error message:
Attempting to root device 01ab756b0330210f ...
restarting adbd as root
Finished rooting device 01ab756b0330210f
Attempting to root device 0ca41989032f9f23 ...
restarting adbd as root
Finished rooting device 0ca41989032f9f23
Attempting to root device 0cb844c5032fdbdb ...
adbd is already running as root
Finished rooting device 0cb844c5032fdbdb
Attempting to root device 0cba90a3032fdbe3 ...
restarting adbd as root
Finished rooting device 0cba90a3032fdbe3
Attempting to root device 0cbc5fcf0c36f1aa ...
restarting adbd as root
Finished rooting device 0cbc5fcf0c36f1aa
Attempting to root device 0cbc67090c371bb4 ...
error: closed
Traceback (most recent call last):
  File "/tmp/tmpBxKYP7.py", line 7, in <module>
    subprocess.check_call([adb_path, '-s', device, 'root'])
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/b/c/b/Android_Tests__dbg_/src/third_party/android_tools/sdk/platform-tools/adb', '-s', '0cbc67090c371bb4', 'root']' returned non-zero exit status 1
step returned non-zero exit code: 1
@@@STEP_LOG_LINE@python.inline@@@@
@@@STEP_LINK@logdog-->python.inline@https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Fchromium.linux%2FAndroid_Tests__dbg_%2F35688%2F%2B%2Frecipes%2Fsteps%2FRoot_devices%2F0%2Flogs%2Fpython.inline%2F0@@@
@@@STEP_LOG_LINE@python.inline@import subprocess@@@
@@@STEP_LOG_LINE@python.inline@import sys@@@
@@@STEP_LOG_LINE@python.inline@adb_path = sys.argv[1]@@@
@@@STEP_LOG_LINE@python.inline@for device in sys.argv[2:]:@@@
@@@STEP_LOG_LINE@python.inline@  print 'Attempting to root device %s ...' % (device)@@@
@@@STEP_LOG_LINE@python.inline@  subprocess.check_call([adb_path, '-s', device, 'root'])@@@
@@@STEP_LOG_LINE@python.inline@  subprocess.check_call([adb_path, '-s', device, 'wait-for-device'])@@@
@@@STEP_LOG_LINE@python.inline@  print 'Finished rooting device %s' % (device)@@@
@@@STEP_LOG_END@python.inline@@@
@@@STEP_FAILURE@@@