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There is a bug in heartbeat sender's backoff logic such that the
backoff is not capped to a maximum retry interval. This CL:

* Fixes the bug by switching to use BackoffEntry
* Adds unittests
* Adds LOG to help future debugging

Change-Id: Idd737d2c8edfec326adf394eef5ee6857d757287
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636829
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664637}