This reverts commit c91686c0.

Reason for revert: Reland a light version of the original CL that keeps a dummy flag around.

Original change's description:
> Revert "Remove obsolete bot_update flag"
> 
> This reverts commit 72048266.
> 
> Reason for revert: Skia depends on it. Need to clean that up first. Currently blocks roll:
> https://skia-review.googlesource.com/c/14145/
> 
> Original change's description:
> > Remove obsolete bot_update flag
> > 
> > Using the manifest is the default since a long time. The flag was only introduced to smoothly switch it on. Now the flag is not needed anymore.
> > 
> > Bug: 398105
> > Change-Id: I4eb1c98e26e025a69f8b0a7c48877d78c031702e
> > Reviewed-on: https://chromium-review.googlesource.com/485659
> > Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
> > Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
> > Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> > 
> 
> TBR=phajdan.jr@chromium.org,machenbach@chromium.org,tandrii@chromium.org,sergiyb@chromium.org,chromium-reviews@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> 
> Change-Id: Iffc4d8076abd3c92feebfd7f3ef88be7ce34b692
> Reviewed-on: https://chromium-review.googlesource.com/486639
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> 

R=phajdan.jr@chromium.org,machenbach@chromium.org,tandrii@chromium.org,sergiyb@chromium.org,chromium-reviews@chromium.org
Bug: 398105

Change-Id: I817d446f8a8eff84c5d7143ac2b9c527a40991f1
Reviewed-on: https://chromium-review.googlesource.com/486640
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>