Add a Mac UMA for fast user switching.

BUG=914393

Change-Id: Ife24dd2508cd912b210299130059d2b818300c27
Reviewed-on: https://chromium-review.googlesource.com/c/1412997
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#623400}(cherry picked from commit 8bcf1fdb0dfc2a870dc093f228b3b208d1184e66)
Reviewed-on: https://chromium-review.googlesource.com/c/1427000
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/branch-heads/3626@{#753}
Cr-Branched-From: d897fb137fbaaa9355c0c93124cc048824eb1e65-refs/heads/master@{#612437}
3 files changed