Builds occasionally fail due to Could not prepare any devices for testing.

This happens on Builder: WebKit Android (Nexus4) 
with, "Could not prepare any devices for testing."


https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Android%20%28Nexus4%29/builds/54282

https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Android%20%28Nexus4%29/builds/54280

https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Android%20%28Nexus4%29/builds/54276




Snippet of log

11:42:34.079 13592 "ruby --version" took 0.06s
11:42:34.082 13592 "/usr/bin/wdiff --help" took 0.00s
11:42:34.121 13592 "/usr/sbin/apache2 -v" took 0.04s
11:42:34.121 13592 [01cb7ce0cfe305ae] preparing device
11:42:34.121 13592 [01cb7ce0cfe305ae] Run adb command: ['shell', 'ps']
11:42:34.322 13592 "/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb -s 01cb7ce0cfe305ae shell ps" took 0.20s
11:42:34.322 13592 [01cb7ce0cfe305ae] Run adb result: USER     PID   PPID  VSIZE  RSS     WCHAN    PC         NAME
root      1     0 
11:42:34.322 13592 [01cb7ce0cfe305ae] Run adb command: ['shell', 'kill', '-15', u'264']
11:42:34.375 13592 "/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb -s 01cb7ce0cfe305ae shell kill -15 264" took 0.05s
11:42:34.375 13592 [01cb7ce0cfe305ae] Run adb result: 
11:42:34.375 13592 [01cb7ce0cfe305ae] Run adb command: ['wait-for-device']
11:42:35.379 13592 "/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb -s 01cb7ce0cfe305ae wait-for-device" took 1.00s
11:42:35.380 13592 [01cb7ce0cfe305ae] Run adb result: 
11:42:35.380 13592 [01cb7ce0cfe305ae] Run adb command: ['root']
11:42:35.428 13592 "/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb -s 01cb7ce0cfe305ae root" took 0.05s
11:42:35.428 13592 [01cb7ce0cfe305ae] Run adb result: restarting adbd as root
11:42:35.428 13592 [01cb7ce0cfe305ae] Run adb command: ['wait-for-device']
11:42:35.432 13592 "/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb -s 01cb7ce0cfe305ae wait-for-device" took 0.00s
11:42:35.432 13592 [01cb7ce0cfe305ae] Run adb result: 
11:42:35.432 13592 [01cb7ce0cfe305ae] Run adb command: ['shell', 'ls', '-d', '/data/local/tmp/md5sum_bin']
11:42:35.436 13592 "/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb -s 01cb7ce0cfe305ae shell ls -d /data/local/tmp/md5sum_bin" took 0.00s
11:42:35.436 13592 [01cb7ce0cfe305ae] failed to prepare_device: Failed to run "['/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb', '-s', u'01cb7ce0cfe305ae', 'shell', 'ls', '-d', '/data/local/tmp/md5sum_bin']" exit_code: 255

output: error: device '01cb7ce0cfe305ae' not found
11:42:35.437 13592 [01cb7ce0cfe305ae] Run adb command: ['shell', 'am', 'force-stop', 'org.chromium.content_shell_apk']
11:42:35.440 13592 "/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb -s 01cb7ce0cfe305ae shell am force-stop org.chromium.content_shell_apk" took 0.00s
Exception webkitpy.common.system.executive.ScriptError: ScriptError(u'Failed to run "[\'/b/c/b/WebKit_Android__Nexus4_/src/third_party/android_tools/sdk/platform-tools/adb\', \'-s\', u\'01cb7ce0cfe305ae\', \'shell\', \'am\', \'force-stop\', \'org.chromium.content_shell_apk\']" exit_code: 255\n\noutput: error: device \'01cb7ce0cfe305ae\' not found\n',) in <bound method ChromiumAndroidDriver.__del__ of <webkitpy.layout_tests.port.android.ChromiumAndroidDriver object at 0x7f17e7f3efd0>> ignored
11:42:35.441 13592 Could not prepare any devices for testing.
11:42:35.441 13592 Build check failed
11:42:35.441 13592 
11:42:35.441 13592 Testing completed, Exit status: 254
11:42:35.441 13592 
11:42:35.441 13592 Process dump:
11:42:35.455 13592 	USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
11:42:35.455 13592 	root         1  0.2  0.0  33636  2900 ?        Ss   11:27   0:02 /sbin/init