Add a Mac UMA for finding a staging directory.

BUG=914393

Change-Id: Ia7ac0137972c45bad9e4a1013f21665bd4fac8a8
Reviewed-on: https://chromium-review.googlesource.com/c/1416370
Commit-Queue: Avi Drissman <avi@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623931}
3 files changed