[Extensions] Remove support for multiple windows for loginScreenUi API

Since the created window has fixed size and is not draggable,
multi-window support does not make sense anymore. We only ever expect
there to be one login screen extension showing UI at a time.

Bug: 957573
Change-Id: I90fcb01d3d8098abe2350c8773df03ce0cdedcfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768643
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Zakhar Voit <voit@google.com>
Cr-Commit-Position: refs/heads/master@{#690357}
7 files changed