Rather than doing parent-document lookup every time you call SetDocumentAndPageUseCounters.

It's a constant, not a global. This matches both the Mozilla and Google coding
styles so should hopefully be uncontroversial.

It's just a bit flag, should be no need of checking whether the bit is set
before.

This is so that SetUseCounter is as cheap as possible.

This is in preparation for hooking the CSS use counters to telemetry. We want
CSS use counters to be fast and be propagated at once to the parent page. This
will make sure to use the same setup as everywhere else.

https://hg.mozilla.org/mozilla-central/rev/a52f74c30955
https://hg.mozilla.org/mozilla-central/rev/fc006441b6d9
https://hg.mozilla.org/mozilla-central/rev/adfc50ea76f4

Comment on attachment 9090352 [details]
Bug 1578700 - Change use counter setup to propagate the page use counters together.

Beta/Release Uplift Approval Request

• User impact if declined: See bug 1578661 comment 51.
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Medium
• Why is the change risky/not risky? (and alternatives if risky): While this is a change to the setup of how we recollect data, our tests do cover various edge cases here, and so far the data in Nightly 71 vs Nightly 70 looks pretty comparable.

So it doesn't seem this patch introduced any logic error that changed what we measure.

• String changes made/needed: none

Comment on attachment 9090350 [details]
Bug 1578700 - Rename a variable to better follow our coding style.

Fix for use counters needed for some other approved patches. Approved for 70.0b10.

https://hg.mozilla.org/releases/mozilla-beta/rev/84589f69c57b
https://hg.mozilla.org/releases/mozilla-beta/rev/cbb2a9654a65
https://hg.mozilla.org/releases/mozilla-beta/rev/2d698478059c