We keep an index hint for the next pending player message. This hint
wasn't updated correctly when messages are removed due to a timeline
update.
This change makes sure to only use the hint locally in one method so
that it doesn't need to be updated anywhere else and also adds the "hint"
suffix to the variable name to make it clearer that it's just a hint and
there are no guarantees this index actually exists anymore.
issue:#7278
PiperOrigin-RevId: 309217614
85bc1d6e14