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This CL aims to update the elevation shadows on a few of the lingering
places where we are using paper's shadows. The added CSS variables
also will allow other WebUI to consume the variables [1].

https://imgur.com/a/8qlbMMn

[1] https://source.chromium.org/search?q=%220%204px%208px%203px%22%20filepath:html&ss=chromium&originalUrl=https:%2F%2Fcs.chromium.org%2F

Bug: 973674
Change-Id: I3eeff199526fc499c405edb39494b526c446f45e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209634Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771958}