Fix wpt/css/css-masking/parsing/clip-valid.html subtest

Percentages are not allowed in rect() (arguments are '<length> | auto'.)
Move the old test to clip-invalid.html and replace it with one that
doesn't use percentages.

Bug: 843084
Change-Id: I3cadd99da0d302ac5a0cd012450dd7ade233681c
Reviewed-on: https://chromium-review.googlesource.com/1071469
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561598}
3 files changed