GitLab

This enables the build on Mac OS X, and refactors
AlternativeBrowserDriver to work correctly on Mac OS.

Bug: 881589
Change-Id: I6e73d33d83af5bc1c0d8bb3ec3e76fcf8409ee2e
Reviewed-on: https://chromium-review.googlesource.com/c/1283835Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601570}