Deflake css3/filters/composited-layer-bounds-after-sw-blur-animation

It is not clear why this test flakes with the wrong class name, but the
test is only concerned with the size of the layers (see:
https://bugs.webkit.org/show_bug.cgi?id=109098), so this patch rewrites
the test to not depend on the class name.

Bug: 1268594
Change-Id: I9061754b856686105bbe1e17946a62e41167af03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4761786
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1181132}