[autotest] add constant AUTOUPDATE_PRESERVE_LOG to be used in autoupdate control file

BUG=chromium:276919
TEST=None

Change-Id: I1338ae96e884b9135cb7841878d4751fcd83dcfe
Reviewed-on: https://chromium-review.googlesource.com/183153
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
1 file changed