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Adds a per-scene incognito blocker. Removes the old blocker.
The new blocker is implemented as a scene agent that monitors the scene
for state changes and checks a new SceneState flag
|incognitoContentVisible| that was previously owned by SceneController.
Removes old tests and adds new ones.
Uses window ordering by windowLevel instead of keyWindow, which is
actually possible even before scene startup.

especially with full-screen video in an incognito tab.

Fixed: 1125969
Test: on iOS 12, 13, 14 devices test that the incognito blocker works,
Change-Id: I3192908c61fecc7d548383dab4d068a05f4c3ef3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404855
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806642}