Fix WPT css/css-masking/mask-svg-content/mask-with-filter.svg

The filter's region is relative to the object bounding box of the <rect>
in the mask. The object bounding box of that <rect> is <49,49 2x2>. Thus
the results of the filter will be clipped to <48.8,48.8 2.4x2.4> since
the default filter region is the object bounding box expanded by 10%.

Since the test wants the stroked shape to be filtered in its entirety,
switch the filter units to "userSpaceOnUse" to avoid the clipping.

Add a new test that tests that the clipping from the filter region is
applied.

Fixed: 1475379
Change-Id: Ib62674a32de3ab457e14a16095103d0bbd4b3df4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4966743
Auto-Submit: Fredrik Söderquist <[email protected]>
Commit-Queue: Philip Rogers <[email protected]>
Reviewed-by: Philip Rogers <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1213648}