Add intrinsicsize to HTML Media elements (img, video)

[ZaneHannanAU]

Experimental intrinsicsize attribute added.
Explainer: https://github.com/ojanvafai/intrinsicsize-attribute

Chrome: Experimental Web Platform features flag, Version >= Chrome 71. https://www.chromestatus.com/feature/4704436815396864

Tested: no
Primarily done through manual JSON.parse and JSON.stringify ... may need changes for standard_track but not certain.

[ZaneHannanAU]

Oh yeah need to add browser versions...

[ZaneHannanAU]

... can wait until full release.

[wbamberg]

Thanks for your contribution!

... can wait until full release.

It's up to you of course, but I think we can update the Chrome browser list to include 71 now, since our list is out of date already (https://github.com/mdn/browser-compat-data/blob/master/browsers/chrome_android.json#L229-L234). (Is 71 the current Nightly? I'm not sure how things like Canary map to the definitions in browsers.json.) That would unblock this PR.

But it would probably be good to update the list of browsers in a separate PR.

A few comments on this PR:

• please keep subfeatures alphabetised
• I don't see any indication here https://www.chromestatus.com/feature/4704436815396864 or here https://github.com/ojanvafai/intrinsicsize-attribute that this is standard_track.
• the feature seems to be behind a flag, so you'll need to add something like this:

"flags": [
  {
    "type": "preference",
    "name": "Experimental Web Platform Features"
  }
]

See for example https://github.com/mdn/browser-compat-data/blob/master/api/Request.json#L538-L545.

[ZaneHannanAU]

Understood, likely will do this evening.

[jpmedley]

Two points. First, always use the anchor form rather than the string form of the flag. The explanation is here. @Elchi3 will be adding it to the linter.

Second, this is behind the #enable-experimental-productivity-features flag. On the commit expand the changes to the svg_image_element.idl and look for RuntimeEnabled=ExperimentalProductivityFeatures.

[Elchi3]

I've marked this as non-standard, inserted the features alphabetically, and fixed the flag information. Looks good to me now.

All comments addressed, thanks for your review!

[ZaneHannanAU]

Thank you, sorry about … completely forgetting