RetriableHttp to sleep between retries

R=sergeyberezin@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2153483002
Cr-Mirrored-From: https://chromium.googlesource.com/infra/infra
Cr-Mirrored-Commit: 97d61b58808647475f4da7573f5ffa5537a33d4d
2 files changed