Make RetriableHttp to be composable

BUG=
R=sergeyberezin@google.com

Review-Url: https://codereview.chromium.org/2128413002
Cr-Mirrored-From: https://chromium.googlesource.com/infra/infra
Cr-Mirrored-Commit: 715fb10959225469c5650b96a7e002805c90253f
2 files changed