This will require two changes:

1. Save the time taken during the update in chromite/lib/auto_updater
2. Write to the chrome perf dashboard in autoupdate_EndToEndTest

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/15d37a29052264456f101b717ccd274cd42ac45c

commit 15d37a29052264456f101b717ccd274cd42ac45c
Author: David Haddock <dhaddock@chromium.org>
Date: Fri Sep 29 06:26:23 2017

[autotest] Save time taken to update.

Write the length of time taken to do the rootfs update.
The simplest way is to just add a timer to the existing perf script.

BUG= chromium:769468 
TEST=autoupdate_EndToEndTest passes and saves the length of time taken.

Change-Id: I3fa0902737498751b8cee12e2334fa9b8f428331
Reviewed-on: https://chromium-review.googlesource.com/691235
Commit-Ready: David Haddock <dhaddock@chromium.org>
Tested-by: David Haddock <dhaddock@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>

[modify] https://crrev.com/15d37a29052264456f101b717ccd274cd42ac45c/server/site_tests/autoupdate_EndToEndTest/autoupdate_EndToEndTest.py
[modify] https://crrev.com/15d37a29052264456f101b717ccd274cd42ac45c/server/site_tests/autoupdate_EndToEndTest/update_engine_performance_monitor.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/0dbfb98de767bbc5ab451c0cab60caac15f1b7e4

commit 0dbfb98de767bbc5ab451c0cab60caac15f1b7e4
Author: David Haddock <dhaddock@chromium.org>
Date: Mon Oct 02 10:03:42 2017

[chromite] Make perf monitoring start after devserver is started.

Currently we start perf monitoring script before the devserver is
started. We are now going to collect running time too so I will move
this script to be closer to the start of the update.

BUG= chromium:769468 
TEST=au and provision still work

Change-Id: I19334064ccb886471424e175e5c78ca3297df340
Reviewed-on: https://chromium-review.googlesource.com/691158
Commit-Ready: David Haddock <dhaddock@chromium.org>
Tested-by: David Haddock <dhaddock@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>

[modify] https://crrev.com/0dbfb98de767bbc5ab451c0cab60caac15f1b7e4/lib/auto_updater.py

https://chromeperf.appspot.com/report?sid=35a7f7a18983a1e5ab2c4fab359ea94176bb8feb640dd02ac9507358ca2c2cef