Embedder VsyncWaiter must schedule a frame for the right time (#29408)

[godofredoc]

• commit: 43561d8
• commit: a2b8946

Pre-launch Checklist

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  writing and running engine tests.
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[flutter-dashboard]

This pull request was opened from and to a release candidate branch. This should only be done as part of the official Flutter release process. If you are attempting to make a regular contribution to the Flutter project, please close this PR and follow the instructions at Tree Hygiene for detailed instructions on contributing to Flutter.

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[zanderso]

lgtm