Add new skip-ad media session action

[beaufortfrancois]

This PR adds a "skip ad" media session action that may be used by media notification and picture-in-picture window controls

Note that one already exists in Android Media Session: https://developer.android.com/reference/kotlin/android/support/v4/media/session/MediaSessionCompat#ACTION_SKIP_AD

We may want to add support for media session actions feature detection. WDYT?

@mounirlamouri

[jernoble]

This looks like a good solution for a common use case. 👍🏻

[phistuck]

"Skip ad" seems highly specific to me. What if it is an introduction pre-roll that can be skipped?
Why not just "skip"?

[jernoble]

To pile on with @phistuck, one use case is Netflix’s “skip intro” button, which functions like a monotonically decreasing “seek forward X” button. Another is Netflix’s “next episode”, which is “next track” with custom text.

I wonder if we could cover more use cases by allowing sites to provide custom label text for these actions rather than providing specific actions for each use case.

YouTube could offer a “nexttrack” action with a { label: “skip ad”} payload, Netflix could offer a “seekforward” action with a { label: “skip intro” } payload, and a “nexttrack” with a { label: “next episode” } payload.

[OrenMe]

Yes, agree as well with @jernoble and @phistuck and I mentioned some of this in
w3c/picture-in-picture#116 (comment)

There are also more cases for this like ad timer to skip and general ad timer till end of ad.
Also, some publishers insist on having “Advertisment” label when playing an ad, either it be regulatory requirement or just brand requirement.

What about ad clickthrough? Adding a “Learn more” button for ads.

Should we open a new issue for these? Are these concerns valid?

[jernoble]

@OrenMe I want to be careful not to go down the slippery slope to "full HTML replacement" here. And in fact, my point was to narrow the API rather than broaden it.

I think the "advertisement" label could be done with a forced subtitle track, if necessary. Ad clickthrough is harder, but I don't think we need to handle that use case explicitly in the Media Session spec. Sites that really, really want to could just add a "nexttrack" action with a { label: 'learn more' } payload. (I don't really like that idea, personally, but they could do it.)