[ios] Fix a snapshot for iOS 26

A snapshot with a translucent element is not taken correctly with
CALayer's `render` method. The method is used for new tab pages and
pages that contain JavaScript dialogs. Newer API to take a snapshot
`drawHierarchy` works well with a translucent element but it doesn't
work well below iOS 26. Use `drawHierarchy` on iOS 26 and after iOS 26
and use the old method below iOS 26.

Using `drawHierarchy` API requires more setup in unit tests. The view
should be added to an UIWindow and rendered on the screen. This CL adds
additional setup to unit tests where a snapshot is taken.

Bug: 444755332, 448839636
Change-Id: Ic4253fc70dd7c8a882d8fc045384753a11ad6576
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7003272
Commit-Queue: Asami Doi <asamidoi@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529565}