GitLab

Remove Windows and Mac configs since work on those platforms
is finished.
Leave Sandbox for Linux as the only feature for experimentation.

Bug: 1019245
Change-Id: I36a141ce875af5813dad14a230a1356beb4af059
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153052Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760396}