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This CL cleans up the layout tests for backdrop-filter:
 - The backdrop-filter-bleeding.html test was never really complete, and
   the effect it was testing for is completely covered by the existing
   external/wpt/css/filter-effects/backdrop-filter-edge-pixels.html
   test.
 - The backdrop-filter-boundary.html test never had a valid expectation
   file. It now does, and the test has been made a bit more obvious. It
   already passed.
 - Another CL (http://crrev.com/663338) made the
   backdrop-filter-clipped.html test pass.

Bug: 497522
Change-Id: Ib95edda9dfb15fc451eaeb225310d422d70787df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637490
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664954}