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In this change, we add support for RetrySystemUpdate. Also, we stop
mapping `RebootSource::OTA` to `StateControlRebootReason::SystemUpdate`
because reboots for system updates should only be called from
fuchsia.git -- the code we remove isn't being run anyway.

BUG=fuchsia:64591
TEST=cast_reboot_unittests

Change-Id: Ic15f86d31d3985d2fa5a2a3957c5ca61066cd476
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2600053Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Zach Kirschenbaum <zkbaum@google.com>
Cr-Commit-Position: refs/heads/master@{#838871}