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This CL adds corner roundedness matching for the reduced motion tab
grid transition animation (which is also used when exiting the tab grid
from a tab that isn't visible).

The free function that exposes the device's (approximate) corner radius
is facored into a ui_utils method.

Bug: 870659
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I5abdd190140f2a4a4fd3b0b18f0d300a6d0dd5ea
Reviewed-on: https://chromium-review.googlesource.com/1177606Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584014}