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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/+/2386535Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803274}