Remove fullscreen popups experimental feature.
Removes the fullscreen popups feature. This is mainly a revert of
crrev.com/c/4072455 and some followup CLs. Removes feature flag,
tests, UMA metrics and all of the navigation "is_fullscreen" plumbing to support the feature.
See: https://chromestatus.com/feature/6002307972464640
Bug: 345222811
Change-Id: I7994a18a8473407050ae5ec0a732df4d73732776
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5617224
Reviewed-by: Mike Wasserman msw@chromium.org
Reviewed-by: Avi Drissman avi@chromium.org
Reviewed-by: Robert Kaplow rkaplow@chromium.org
Reviewed-by: Nate Chapin japhet@chromium.org
Commit-Queue: Brad Triebwasser btriebw@chromium.org
Reviewed-by: Elly FJ ellyjones@chromium.org
Cr-Commit-Position: refs/heads/main@{#1313712}