Mark http/tests/misc/slow-loading-image-in-pattern.html as flaky timeout on retina

Bug: 853360
Change-Id: I016ec6669df8cb5c198d6a8a4151356aa3374f52
Tbr: xiaochengh@chromium.org
NoTry: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1984679
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727825}

third_party/blink/web_tests/TestExpectations

@@ -5014,6 +5014,7 @@ crbug.com/851090 [ Linux ] virtual/mouseevent_fractional/fast/events/middleClick
 
 # Flaky timeout only on Mac 10.13.
 crbug.com/853360 [ Mac10.13 ] http/tests/misc/slow-loading-image-in-pattern.html [ Timeout Pass ]
+crbug.com/853360 [ Retina ] http/tests/misc/slow-loading-image-in-pattern.html [ Timeout Pass ]
 
 # Origin Policy: Skip tests that rely on --feature-enabled=OriginPolicy, so
 #   they can be run via virtual/origin-policy instead.