Disabling flaky backdrop-filter-plus-filter test

TBR=masonfreed@chromium.org
NOTRY=true

Bug: 912748
Change-Id: Ifd814b53adc325737cc5f688551e6fe9c64d910f
Reviewed-on: https://chromium-review.googlesource.com/c/1368164Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614736}

third_party/blink/web_tests/TestExpectations

@@ -1918,6 +1918,8 @@ crbug.com/497522 external/wpt/css/filter-effects/backdrop-filter-paint-order.htm
 crbug.com/497522 external/wpt/css/filter-effects/backdrop-filter-isolation-fixed.html [ Failure ]
 crbug.com/497522 external/wpt/css/filter-effects/backdrop-filter-edge-pixels.html [ Pass Failure ]
 crbug.com/497522 external/wpt/css/filter-effects/backdrop-filter-isolation.html [ Failure ]
+# Flaky timeout: https://crbug.com/912748
+crbug.com/912748 external/wpt/css/filter-effects/backdrop-filter-plus-filter.html [ Skip ]
 # ====== Backdrop-filter related tests END ======