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This CL updates the New Tab button to have fixed positioning. This
CL also updates the scroll animation so that it takes the edge of
the New Tab button as the right-most boundary, instead of the
edge of the viewport.

Bug: 1032650
Change-Id: If656545b5d4b4a50197a5f0d94baeb26b6f7df09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1977841Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726986}