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This CL sets up the scroll container to the the TabListElement itself,
allowing the TabListElement to directly control its own scroll
position without having to modify anything outside of itself. This
also allows for various optimizations so that layout does not need
to be re-calculated multiple times.

Other optimizations include using a cache of CSS variables to
determine the relatively static dimensions of tabs and viewport.

Bug: 1023492
Change-Id: I00874f36ab022aed15477c98652f8014b6098ce8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948059Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721770}