GitLab

- Resurrects shell_dialogs_unittests on Mac
- Fixes 2 tests that had regressed since the gn transition.
- Tweaks the code to be more robust around complex object lifetimes.
- Adds test SelectFileDialogMacTest.Lifetime

Bug: 788271
Change-Id: Ice43179a1da3b7bbcd44fb9b0efdf6d84e4d2f5b
Reviewed-on: https://chromium-review.googlesource.com/920961Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537419}