2/3 Allow devserver RPC to use arbitrary payload file for update.

This add support for the payload_filename to the cros_au RPC and transports it
to chromite/lib/auto_updater.py.

BUG=chromium:709710
TEST=autoupdate_EndToEndTest

Change-Id: I22657038f57d84ba46c65388d5fe3c21759ab72a
Reviewed-on: https://chromium-review.googlesource.com/471882
Commit-Ready: Hsinyu Chao <hychao@chromium.org>
Tested-by: David Haddock <dhaddock@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
2 files changed