[omnibox][cr23] Fix omnibox border being too thin due to UI scaling.

Currently, the omnibox border appears too thin on devices that make use
of UI scaling (e.g. Mac with Retina display). As such, this CL updates
the relevant code to properly scale the border based on the device
scaling factor.

NOTE: All below screenshots were taken with high-contrast mode enabled.

  * unfocused - BEFORE: http://screen/32TMnFVPj7KbApq
  * unfocused - AFTER:  http://screen/8ydR3ZGp4DvRdqw

  * focused - BEFORE: http://screen/B4K5abBx6UVCR5r
  * focused - AFTER:  http://screen/BABYEhafQHNktE6

Bug: 1476659
Change-Id: I59cbf78eef6808ead4f2bd090bb23336ebf9ce27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4850200
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Commit-Queue: Khalid Peer <khalidpeer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1193774}