fix: do not run onStage listeners for stages that were aborted (#95684)
Small fix for `StageRenderingController.onStage` -- the `StageTrigger`
implementation that `onStage` is based around would immediately fire the
callback if the stage trigger was cancelled (i.e. if we aborted the
render and won't reach the stage). The intention was to only fire it
synchronously if the trigger was fired (i.e. we advanced to the stage)
Luckily it looks like we have literally one usage of `onStage` right
now: in `countShellAndStaticStageBytes`, which is only used in full
renders that don't abort, so this shouldn't be a factor, but still worth
doing for future-proofing