libchromeos: Add utility class for exponential backoff mechanism

Borrowed the implementation of BackoffEntry class from the inner workings
of Chromium (from net/base/backoff_entry.*).

This is a very helpful class to manage retry delay management with
exponential backoff and fuzzing spread.

BUG=brillo:955
TEST=`FEATURES=test emerge-link libchromeos`

Change-Id: Idfa9143c63358a7227ed279b44a9fda39a1597d1
Reviewed-on: https://chromium-review.googlesource.com/268443
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
3 files changed