This is supported by gclient and has the effect of treating the solution
as a purely local, standalone DEPS (i.e. not as a source repo to
checkout or process in any way). It also prevents the solution from
being added to the output of 'gclient flatten'.

R=agable@chromium.org, phajdan.jr@chromium.org

Change-Id: Ie11246ec7a74097b96ea6a26983a63b3fb3123a5
Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build
Reviewed-on: https://chromium-review.googlesource.com/698271
Commit-Queue: Michael Moss <mmoss@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>