does it involve any functional change? or it's just a name change?

I got it. It's functional change. I've found it in testharness.js API.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7ae79f6845ea45bb6bc691dec8614ce85b302e1f

commit 7ae79f6845ea45bb6bc691dec8614ce85b302e1f
Author: Liquan (Max) Gu <maxlg@chromium.org>
Date: Tue Aug 01 01:17:37 2017

Long tasks: replace settimeout with step_timeout in test and fix the test

Replace settimeout with set_timeout in the test longtask-in-parentiframe.html,
according to testharness-api.
The original test fails because the long task is fired before the observer is
created, so this CL fixes it by delaying the long task until the iframe is
loaded.

Bug:  745031 
Change-Id: I89e0f0fab5ef0458c773f90ded428fc1a483913c
Reviewed-on: https://chromium-review.googlesource.com/579593
Reviewed-by: Shubhie Panicker <panicker@chromium.org>
Commit-Queue: Liquan Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490831}
[modify] https://crrev.com/7ae79f6845ea45bb6bc691dec8614ce85b302e1f/third_party/WebKit/LayoutTests/external/wpt/lint.whitelist
[modify] https://crrev.com/7ae79f6845ea45bb6bc691dec8614ce85b302e1f/third_party/WebKit/LayoutTests/external/wpt/longtask-timing/longtask-in-parentiframe.html
[modify] https://crrev.com/7ae79f6845ea45bb6bc691dec8614ce85b302e1f/third_party/WebKit/LayoutTests/external/wpt/longtask-timing/resources/subframe-observing-longtask.html