https://build.chromium.org/p/chromium.android/builders/Lollipop%20Tablet%20Tester/builds/8287

https://chromium-swarm.appspot.com/task?id=36ac25a7f2b3bc10&refresh=10&show_raw=1

Says that all tests pass (and almost all other bots are happy), but still exits with an error:

I  173.445s run_tests_on_device(0a294402)  [  PASSED  ] 256 tests.
I  173.445s run_tests_on_device(0a294402)  <<ScopedMainEntryLogger
I  173.448s TimeoutThread-1-for-run_tests_on_device(0a294402)  [host]> /b/swarming/w/ir/third_party/android_tools/sdk/platform-tools/adb -s 0a294402 shell '( su -c sh -c '"'"'test -e /data/tombstones'"'"' );echo %$?'
I  173.512s TimeoutThread-1-for-run_tests_on_device(0a294402)  [host]> /b/swarming/w/ir/third_party/android_tools/sdk/platform-tools/adb -s 0a294402 shell '( su -c sh -c '"'"'TZ=utc ls -a -l /data/tombstones/'"'"' );echo %$?'
W  173.612s run_tests_on_device(0a294402)  No tombstones to clear.
I  173.613s TimeoutThread-1-for-run_tests_on_device(0a294402)  [host]> /b/swarming/w/ir/third_party/android_tools/sdk/platform-tools/adb -s 0a294402 shell '( pm dump org.chromium.native_test | grep dataDir=; echo "PIPESTATUS: ${PIPESTATUS[@]}" );echo %$?'
ERROR:root:Forwarding signal 15 to test process
[W2017-06-10T19:41:54.467625Z 4491 0 butler.go:240] Butler Context was cancelled. Initiating shutdown. {"error":"context canceled"}
[E2017-06-10T19:41:54.469573Z 4491 0 subcommand_run.go:333] Command completed with non-zero return code. {"returnCode":-1}
[E2017-06-10T19:41:54.469701Z 4491 0 main.go:205] Butler terminated with error.               {"error":"context canceled"}
[E2017-06-10T19:41:54.469722Z 4491 0 main.go:233] Error running bootstrapped Butler process:
original error: context canceled

[I2017-06-10T19:41:54.469756Z 4491 0 main.go:349] Terminating.                                {"returnCode":-1}


Very similar for 3 different test binaries on Marshmallow Tablet Tester: https://build.chromium.org/p/chromium.android/builders/Marshmallow%20Tablet%20Tester/builds/8696