[SxS] Move watermark view to contents container view.

This change allows watermarks to be applied to both contents in a split
view. The change introduces a browser window feature which applies
watermark based to individual watermark views based on their associated
web contents which is tracked by the tab feature.

BYPASS_LARGE_CHANGE_WARNING

Bug: 406793514
Change-Id: Ic33467dce91d52f66a23a355e81f6874887950a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6788777
Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Eshwar Stalin <estalin@chromium.org>
Reviewed-by: David Pennington <dpenning@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1500571}