Revert "Remove unnecessary looping over all root windows"

This reverts commit 822d38eacde7849678cc7cca74b75d0c5f4ed773.

Reason for revert: Breaks window magnetism when dragging to an external display.

Original change's description:
> Remove unnecessary looping over all root windows
> 
> We don't need to check magnitism between windows
> on different roots/displays. We should just use
> the same root of the window being dragged.
> 
> BUG=942728
> 
> Change-Id: Iffbdcc9fef29c84fe090a8c50dde88e4a387dc2f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1526632
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#641370}

TBR=oshima@chromium.org,afakhry@chromium.org

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

Bug: 942728
Change-Id: If557ff47c288dd3b091aa2a80c81b7b0cd66481f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529204
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641731}
1 file changed