fix(focus-origin): focus origin sometimes invalid in firefox 57

[devversion]

• Sometimes the focus origin is not valid because Firefox seems to focus a tick after the keyboard/interaction event fired. To ensure the focus origin is always correct, the focus origin will be added to the beginning of the event loop in the next tick.

Fixes #6984

[mmalerba]

needs rebase

[mmalerba]

@devversion please rebase, re-apply merge ready when done

[devversion]

@mmalerba Rebased. I didn't apply merge ready because I'm not sure whether we still want to go that way.

[mmalerba]

I think we should try, it may wind up causing too many failures in google3 to be feasible. I'll mark it merge ready so the caretaker can at least try though

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "continuous-integration/travis-ci/pr" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[mmalerba]

@devversion it looks like we did presubmit this a while back and it seemed fine, it has a failing test on travis though

[devversion]

@mmalerba Should be fixed now.

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