[perf] Remove border radius on tabs panel.
[perf] Remove border radius on tabs panel.

Screenshot of thing being removed:
  https://screenshot.googleplex.com/fym7fkEr6Af.png

Change-Id: I3552986ece75b692f1592398ee13599581ad41fd
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/268819
Auto-Submit: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot