Fix message loop behavior when host callback is cancelled

[gaearon]

c2a1a77

New test.

This works on master but would have been broken by #16145.
I verified the test fails on that branch.

3b70481

This is the fix. Again, it doesn't affect master because we don't use cancelHostCallback yet. But it fixes the previously added test when combined with #16145. The actual problem isn't related to #16145, we just didn't execute the code path that triggers the bad case until that PR.

The problem itself is that cancelHostCallback would clear out the callback. Then in the message loop we would see that it's null, and exit early. But we wouldn't schedule another one, so the loop would stop. However, the boolean itself would stay true, so next time we schedule something, we'd think the loop was still running. And fail to schedule it. The fix is to reset the boolean.

The rAF codepath handles this earlier — in onAnimate. But we don't have that entry point in the message loop implementation. So we forgot to do it.

22a1287

Just some more tests from #16271 since we're not doing it for now.

[sizebot]

No significant bundle size changes to report.

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