[fuchsia] MaybeRunInitialVsyncCallback should only called once

[chaopeng]

Currently, flutter keeps calling MaybeRunInitialVsyncCallback() until 1 OnNextFrameBegin() called from Fuchsia which maybe problem when display is off.

Tested manually.

Bug: http://fxbug.dev/376079469

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[chaopeng]

@uysalere