Looked into a stdout of an example failed build: https://build.chromium.org/deprecated/chromium.gpu/builders/Win10%20Debug%20(NVIDIA)/builds/2120/steps/steps/logs/stdio. It failed when running //build/scripts/slave/logdog_bootstrap.py, the command step is https://codesearch.chromium.org/chromium/build/scripts/slave/logdog_bootstrap.py?l=601. This is the error message:

INFO:remote_run:Executing command: ['E:\\b\\.recipe_cipd\\logdog_butler.exe', '-log-level', 'warning', '-project', 'chromium', '-prefix', 'bb/chromium.gpu/Win10_Debug__NVIDIA_/2120', '-coordinator-host', 'logs.chromium.org', '-output', 'logdog', '-tag', 'buildbot.master=chromium.gpu', '-tag', 'buildbot.builder=Win10_Debug__NVIDIA_', '-tag', 'buildbot.buildnumber=2120', '-tag', u'logdog.viewer_url=https://luci-milo.appspot.com/buildbot/chromium.gpu/Win10 Debug (NVIDIA)/2120', '-service-account-json', 'c:\\creds\\service_accounts\\service-account-luci-logdog-publisher.json', '-output-max-buffer-age', '30s', '-io-keepalive-stderr', '5m', 'run', '-stdout', 'tee=stdout', '-stderr', 'tee=stderr', '-streamserver-uri', 'net.pipe:LUCILogDogButler', '--', 'E:\\b\\.recipe_cipd\\logdog_annotee.exe', '-log-level', 'warning', '-name-base', 'recipes', '-print-summary', '-tee', 'annotations', '-json-args-path', 'E:\\b\\rr\\tmpzfhmsz\\logdog_annotee_cmd.json', '-result-path', 'E:\\b\\rr\\tmpzfhmsz\\bootstrap_result.json']

[W2018-04-19T07:01:59.729354-07:00 3548 0 client.go:306] RPC failed transiently. Will retry in 1s    {"error":"failed to send request: Post https://logs.chromium.org/prpc/logdog.Registration/RegisterPrefix: net/http: TLS handshake timeout", "host":"logs.chromium.org", "method":"RegisterPrefix", "service":"logdog.Registration", "sleepTime":"1s"}
...


I have no knowledge about the 'RPC' error at the client.go side. CCing dnj@ and iannucci@ who are more familiar with logdog_bootstrap.