Reland "Storage Monitor: Port Mac ImageCaptureDevice off of FILE thread."

This is a reland of a456fdbdbac4831652a01e084ab512b8b6919619
Original change's description:
> Storage Monitor: Port Mac ImageCaptureDevice off of FILE thread.
> 
> Bug: 575383
> Change-Id: I7e2908ff4ffb93fe30e48a54e6c0ab616ce1f736
> Reviewed-on: https://chromium-review.googlesource.com/588078
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Commit-Queue: Tommy Li <tommycli@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#490102}

Bug: 689520
Change-Id: Ibe5a7b95d6ef2de1b452a42d0a1b4435f3083721
Reviewed-on: https://chromium-review.googlesource.com/591829
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490525}
3 files changed