Reland "update ScrollMetricsNotification (#87421)"

[chunhtai]

This reverts commit 0f465e5.

previous pr broke internal tests. The test will be fixed by cl/388794349

blocked on cl/388794349

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[chunhtai]

cc @Piinks @xu-baolin

[xu-baolin]

Some tests failed because of multiple assertions reported.

I am surprised that the test did not report an error when it first landing.

[chunhtai]

7bbcdc239b changed the way the error is reported

[Piinks]

It looks like the failing tests reveal a condition that should be added to the changes from 7bbcdc2
If more than one position is attached to the controller, then RawScrollbarState._shouldUpdatePainter should return false. The failing checks appear to be from the gallery integration tests.
@chunhtai do you want to add that here? Or should I add it in a separate change?

[chunhtai]

@Piinks I will add it in this pr

[Piinks]

LGTM!

[Piinks]

Ooo this is a very exciting change. There are so many work arounds for this that we could clean up. :)

[chunhtai]

Test run https://fusion2.corp.google.com/presubmit/tap/389684836/OCL:389684836:BASE:389709544:1628540380244:3438f481

one golden failure that is a pretty minor diff and is probably the correct change merging this pr