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Remove the special shape of the collapse button. Changing it based on
the dark mode's spec.

See recorded video at https://drive.google.com/file/d/1jUzzACXKYKjX-VLtNJMM5PrPPaJ9Pv0g/view?usp=sharing&resourcekey=0-h-Z7-5Ul0ub5kc8HN15FIQ

Bug: 1145377
Change-Id: If00bffe750df0adf19af9888a6362f5d461c6514
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566896Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832460}