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The crash was because a new screenshot is requested when the current
screenshot is about to get scrolled out of bounds. It was using a fixed
number threshold, meaning displays that are not the same size could
get out of sync easily, leading to a CHECK failure.

This CL addresses that by requesting screenshot on all displays if even
one of them needs one. Another approach would be change the spacing and
threshold to be based on the display width instead of fixed number, but
that could be prone to roundoff error which may cause a similar issue.

Also noticed visible desk metric was double counting when there are
multiple displays. Added a TODO for that one as it is lower priority.

Bug: 1154868
Test: manual, added regression test
Change-Id: I48f5ef62d3284b710f44a2808f75c95686bc9b3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570551
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833993}