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This reverts commit e413add4.

Reason for revert: This approach is wrong. A much simpler approach is now being implemented in 
https://chromium-review.googlesource.com/c/chromium/src/+/2391798

Original change's description:
> [ios] Add ScreenTime overlay modality
> 
> This is one of a series of CLs to implement ScreenTime.
> 
> This CL:
> * Adds a new overlay modality above the web content area for the
> ScreenTime blocking view overlay
> * Adds and integrates no-op supported overlay coordinator classes.
> 
> Bug: 1123704
> Change-Id: Idee3417df9dee7cd15b8fcc93e4ee6944a5e530e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386535
> Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
> Commit-Queue: edchin <edchin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#803274}

TBR=michaeldo@chromium.org,edchin@chromium.org,thegreenfrog@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1123704
Change-Id: If19b96825b7788cbdaa77b8e505278d049807814
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390839Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804470}