GitLab

Remove the now launched default browser fullscreen promo experiment
and add a new feature flag for fullscreen promo UI experimenting.

Bug: 1155778
Change-Id: Id5177c59cb02f355df5e5e806da33d5d9ee51c3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575902
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835801}