Add the ability to share files

[marcoscaceres]

Add a files member to ShareData dictionary.

@ericwilligers, I tried to add in what is in level 2 to kick off discussion.

For normative changes, the following tasks have been completed:

• Modified Web platform tests

Implementation commitment:

• WebKit
• Chromium - Already implemented.
• Gecko

[marcoscaceres]

I think this should be a sequence, no? We wouldn't expect a developer to pass in a frozen array (i.e., {files: Object.freeze(filesArray)}).

Suggested change

	FrozenArray<File> files;
	sequence<File> files;

[marcoscaceres]

@hober, during TPAC you mentioned that this may be of interest to the WebKit community. Would you be the right person to help us with reviewing/designing this extension to web share?

[othermaciej]

There's code implementing file sharing now in WebKit trunk, see: https://bugs.webkit.org/show_bug.cgi?id=207491

[marcoscaceres]

@othermaciej, that's fantastic news! thanks for the update.

Were you folks ok with the way .canShare() works? I'd made an alternative proposal in #108? Happy to close it that if WebKit will follow Chrome's behavior tho.

[othermaciej]

We've currently implemented more or less what is in the spec. However, it's not too late to change what we have before it ships in a non-preview release.

I personally think the change proposed in issue #108 would be an improvement. I think canShare at one point checked for support of sharing specific file types, but now effectively just checks whether file sharing is supported. It seems unwieldy to construct a full ShareData just to check that. One downside is that a method with the same name but different parameters would be maximally likely to cause a compat issue if any content has adopted this AP. I'll comment to that effect in the issue.

[marcoscaceres]

Sent a PR to WPT so the corresponding test is no longer be tentative.

Filed Gecko bug. I'll see if I, or someone on Mozilla's DOM Team, can find time to implement it.

[marcoscaceres]

@ericwilligers, @mgiuca, this is probably ready to land I think.