Merge remote-tracking branch 'remotes/cros/update_engine_aosp_merge'

In the follow up CLs we have to Reland:
eaad5d0f Do not merge to AOSP: Fixes the link to brillo-clang-format in CrOS
740efad4 Reboot even if a system update is not available.
4b0d6032 Add functions to allow update over cellular (including tethered connection)
840703a4 Fix update over cellular network on guest account
Because they have been reverted in cros/upstream branch.

Also we have to partially revert:
2b9d2417  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