Fix flakiness of external/wpt/css/css-position/z-index-blend-will-change-overlapping-layers.html

Use reftest-wait to avoid the test finishing too early.

Change-Id: I2f568e1b4fe55eccec9f07012d7c7f8f6b66fd15
Reviewed-on: https://chromium-review.googlesource.com/c/1491135
Auto-Submit: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636069}
1 file changed