Fix #1224 - only set non-empty MediaCapabilities on MediaKeySystemConfiguration

[davemevans]

It seems that passing [] as a MediaKeySystemMediaCapability in MediaKeySystemConfiguration causes the requestMediaKeySystemAccess call to fail with NotSupportedError.

See #1224 and discussion on Slack today (19/9/2016) for background.

This PR checks whether the capabilities are empty and does not include them in the configuration object in that case.

I have tested this on Chrome with both clearkey and widevine. It'd be great if someone could take a look with other UA/CDM instances.

[dsparacio]

LGTM - liking the new review tools.

[sandersaares]

EME defines an empty list there as the default value, so treating a missing value as different sounds like a browser defect - I suggest filing a bug with the browser vendors, as well, to help get this corrected at the origin.

[davemevans]

FWIW:

• Be explicit about how the lack of audioCapabilities/videoCapabilities in a configuration is handled w3c/encrypted-media#178 (comment)
• shaka-project/shaka-player@5102f54

[davemevans]

I've requested clarity from one of the EME editors. If I get a response I'll reply here and possibly take other actions as suggested by Sander.

In the meantime, I'm going to merge.

[davemevans]

I received a helpful reply from David Dorwin who confirmed that the spec is correct and that we should expect this to work without the PR, but that the current implementation in Chrome was not up to date with the spec. A fix has landed in Canary (and Firefox Nightly I noticed) to implement this correctly.

If you try http://jsbin.com/roxeceteyu/edit?html,output in either Chrome Canary (55) or Firefox Nightly (52), you should see PASS, showing compliant behaviour.

The PR does no harm, its output remains compliant, and it will be required until the nightly browser versions become the stable versions, so I think we should keep it.