✨Update storyNextUp to auto-advance on all pages.

[zaparent]

• Remove isLastPage_() and isLastStoryPage_
• Add getFirstVideo_()
• Update maybeSetStoryNextUp_() to apply to all pages
• Update maybeSetStoryNextUp_() to prefer using the first video when possible
• Add a comment explaining that "storyNextUp" is a param used for testing that may be changed or removed without notice

[amp-owners-bot]

Hey @gmajoulet, @newmuis, @Enriqe! These files were changed:

extensions/amp-story/1.0/amp-story-page.js

[Enriqe]

Thanks for the PR @zaparent :) Do you mind adding a couple of unit tests please?

You can grab inspiration from these test files:

Advancement related:
https://github.com/ampproject/amphtml/blob/main/extensions/amp-story/1.0/test/test-page-advancement.js#L36

story-page and advancement:
https://github.com/ampproject/amphtml/blob/main/extensions/amp-story/1.0/test/test-amp-story-page.js#L137

Checking for a viewer param:

amphtml/extensions/amp-story/1.0/test/test-amp-story.js

Lines 116 to 120 in 6f57dc0

	const viewer = Services.viewerForDoc(env.ampdoc);
	env.sandbox
	.stub(viewer, 'hasCapability')
	.withArgs('swipe')
	.returns(hasSwipeCapability);

[gmajoulet]

@Enriqe thank you for pushing for tests!

We have two followup PRs to make this one work:

• Also auto-advance ads
• Handle short videos (duration < 2s)

The second one will be rather complex and will change most of this implementation. Let's require unit tests for this PR, and unblock this one as the implementation (and the tests) would change.