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This CL makes the |ContinueBrowsingChip| focusable during tab traversal
and adds a focus ring for it with a desired shape.

Screenshot: https://screenshot.googleplex.com/8FbCDLZShLMsSUe.png

BUG=1106937,1126208

Change-Id: I2aff8afe936de2b6861dd1568ae17200f3b34d71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451323Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814419}