Merge remote-tracking branch 'goog/upstream-master'.

The following commits were reverted:
840703a4 Fix update over cellular network on guest account
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.

Fixed a few sign compare warnings.
Had to ifdef out 2 SquashfsFilesystemTest because it depends on unsquashfs -m.

Test: update_engine_unittests
Change-Id: I6f4ca5003e78c76064ec60d0797505d8c18d00bf
Merged-In: I6f4ca5003e78c76064ec60d0797505d8c18d00bf