Allow filtering mutation executions on user-visibility

[dreamofabear]

Enables enforcement user gesture constraints in use cases like amp-script.

Context: ampproject/amphtml#26375

[kristoferbaxter]

Instead of preventing the mutation within each executor, could we stop execution here?

[dreamofabear]

Yea, it's not the prettiest... we do need the return value of execute() for the next start position however.

I also tried adding a new interface method nextPosition(): number that just returns the next start position:

if (allow) { 
  operationStart = executor.execute(mutationArray, operationStart);
} else {
  operationStart = executor.nextPosition(mutationArray, operationStart);
}

But that resulted in a lot of code duplication for more complex command executors, like here:

worker-dom/src/main-thread/commands/event-subscription.ts

Lines 209 to 217 in b837514

	const addEventListenerCount = mutations[startPosition + EventSubscriptionMutationIndex.AddEventListenerCount];
	const removeEventListenerCount = mutations[startPosition + EventSubscriptionMutationIndex.RemoveEventListenerCount];
	const addEventListenersPosition =
	startPosition + EventSubscriptionMutationIndex.Events + removeEventListenerCount * REMOVE_EVENT_SUBSCRIPTION_LENGTH;
	const endPosition =
	startPosition +
	EventSubscriptionMutationIndex.Events +
	addEventListenerCount * ADD_EVENT_SUBSCRIPTION_LENGTH +
	removeEventListenerCount * REMOVE_EVENT_SUBSCRIPTION_LENGTH;

[kristoferbaxter]

Good point, didnt remember that detail.