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This change also updates read later list item button containers to set
up for keyboard navigation. By updating the width, overflow, and
icon-button-fill-color, the buttons can still exist so that backward
keyboard navigation works correctly while not being visible or taking up
space when not hovered.

Bug: 1109316
Change-Id: I0cdc9f7ad5191b2872962e5d4e08024eeada733c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461936Reviewed-by: Connie Wan <connily@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815910}