step_timeout for "noopener" subresources

[annevk]

This is vaguely related to #10911.

There's a class of tests where you have a need for a timeout in a window that doesn't have access to the test instance. It would still be good for that window to use a slower timeout as Cloud infrastructure might require.

For this specific use case it might be good if this function was available in a standalone library you could pull in.

(I've seen folks pull in /resources/testharness.js instead, but while that works it will also lead to the subresources being a test as I understand it, which is not good.)

cc @jgraham

[jgraham]

Also pulling in testharness.js doesn't work and will end up with the multiplier always set to 1 i.e. it's a no-op compared to just using setTimeout directly. So a better solution is needed here.

[foolip]

#5547 is relevant as well.