Add a flag to enable open in window for hosted apps on Mac.

Thie CL adds --[enable|disable]-hosted-apps-in-windows, which controls
whether or not open in windows is allowed for hosted apps. The flag
allows us to fix several outstanding issues with hosted apps on Mac
before enabling open in windows by default.

BUG=517682

Review URL: https://codereview.chromium.org/1300763002

Cr-Commit-Position: refs/heads/master@{#344406}
16 files changed