Merge remote-tracking branch 'remotes/cros/update_engine_aosp_merge'
In the follow up CLs we have to Reland:
eaad5d0f17a1b9d5652a350e1c785c7c5368cb54 Do not merge to AOSP: Fixes the link to brillo-clang-format in CrOS
740efad439e57e71d8d459c366edea3a90b19576 Reboot even if a system update is not available.
4b0d6032cbb86ce488c03b31936cda31283f97e3 Add functions to allow update over cellular (including tethered connection)
840703a4cc77228e2606f45665ae3a4bd75ff7dd Fix update over cellular network on guest account
Because they have been reverted in cros/upstream branch.
Also we have to partially revert:
2b9d2417722cd4052b0e22494886f93c5b4ef042 update_engine: Update libchrome APIS to r456626.
Because we have not upreved libchrome
BUG=chromium:815356
TEST=unittests
TEST=cl with squashfs merge passes
Change-Id: I0ec7f3fe3fd278ef1214868b42d3eb126ebeefe2