[chromecast] Put rounded window corners behind a flag

  * Rounded corners were mistakenly being applied on all classes of
    devices

  * This puts them behind a flag so they can be toggled per device.

Bug: internal b/165638935
Test: manual on device
Change-Id: Ic380d45ba9b2ad53789dbad2acf3860bdea57be5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515039
Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825846}