DisableExtensions now uninstalls them

This is to remain consistent in how the Chrome Cleanup Tool removes other force installed extensions. They interact badly with administrator permissions and so removing them entirely is cleaner than simply disabling like we do with normally installed extensions when the cleanup tool detects UwS.

Change-Id: I1b6b814d521e78edccd7c603f1eae3ef7a9a6c5f
Reviewed-on: https://chromium-review.googlesource.com/c/1278852
Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Preston Carpenter <timidger@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#600059}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 29c8bbfe2e81471ba503af1c03015f0315089b93