Use --enable-experimental-web-platform-features for chrome-dev on Taskcluster

[foolip]

Results from Taskcluster are now being submitted to wpt.fyi staging:
https://staging.wpt.fyi/test-runs?labels=taskcluster,experimental

However, unlike the Buildbot runs, they don't have experimental web platform features enabled.

[jgraham]

There's basically three possible ways to do this; either pass in --binary-arg=--enable[…] in the taskcluster shell script (although note that #12657 removes this), or enable it by default in wptrunner (maybe only if the channel is specified as dev), or in the wpt run frontend (again if the channel is specified as something nightly. For Firefox we effectively do this in wptrunner because we haven't yet split up the prefs we apply into "ones that are actually required to get reliable test results" from "ones that enable experimental features", but the constraints for Chrome are a little different.

[foolip]

@Hexcles, do you have preferences?

Any of those seem fine to me, with perhaps a slight preference for putting it in the Taskcluster setup, because dev without the extra features is a real thing and it'd be odd if it's not even possible to run that.

[jgraham]

It is a real thing, but if Chromium developers are running wpt via wpt run, I assume the most useful default would be to run with experimental features enabled, matching wpt.fyi. Making them pass a flag like --binary-arg=--enable-experimental-web-platform-features to reproduce those results seems unfriendly.

Another options is to add --chrome-experimental and --no-chrome-experimental flags in wptrunner and default to enabled if no specific flag is passed and the channel is dev, or something).

[jugglinmike]

gh-10452 proposes a generic flag that could enable experimental features for any browser