Fix invalid WeakPtr usage in UIDartState::GetSkiaUnrefQueue() #42946
Description
UIDartState::GetSkiaUnrefQueue() is called on threads other than the IO thread, but the WeakPtr used inside there requires us to be on the IO thread.
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UIDartState::GetSkiaUnrefQueue() is called on threads other than the IO thread, but the WeakPtr used inside there requires us to be on the IO thread.