Use task scheduler in chrome_browser_main_mac.mm.

This CL removes the usage of the FILE thread from chrome_browser_main_mac.mm
and resolves an old TODO to make a metadata file only write on Chrome's
first run.

Bug: 667892
Change-Id: I980776c1eeec13a711a74051ac3a436b4b9c5025
Reviewed-on: https://chromium-review.googlesource.com/587703
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490231}
1 file changed