Fix rounded corner backdrop-filter border under zoom

Prior to this CL, the rounded corners on the backdrop filter crop
rect (the rounded corner border box) were not being properly
calculated when zoom was applied. In that case, the corner radii were
being double-scaled by the zoom factor. This fixes that problem
and simplifies the code. This CL also adds a specific test for the
border radius, which I confirmed fails before this patch.

Bug: 979383
Change-Id: Ifc5900ef19ee538bdc4a787e775eafab5683f7ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1683664
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#674122}