Flakiness dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=external%2Fwpt%2Fcss%2Fcss-backgrounds%2Fbackground-image-first-line.html

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ac5da8185581becf26bbe87287ff77963264ec2c

commit ac5da8185581becf26bbe87287ff77963264ec2c
Author: Xianzhu Wang <wangxianzhu@chromium.org>
Date: Wed Jan 16 02:32:24 2019

Deflake external/wpt/css/css-backgrounds/background-image-first-line.html

Wait another frame before ending the test because blink sometimes needs
another frame to update the background-image of ::first-line.

Bug:  922079 
Change-Id: I694f9450b386fb8c50a8becb7ba52ec78e47b910
Reviewed-on: https://chromium-review.googlesource.com/c/1412818
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623055}
[modify] https://crrev.com/ac5da8185581becf26bbe87287ff77963264ec2c/third_party/blink/web_tests/TestExpectations
[modify] https://crrev.com/ac5da8185581becf26bbe87287ff77963264ec2c/third_party/blink/web_tests/external/wpt/css/css-backgrounds/background-image-first-line.html