Allow remote playback device to choose the remote playback source.

[markafoltz]

This addresses Issue #125: RemotePlaybackState enum can become misleading when changing media.src.

It clarifies that:

1. All media resources in a media element are eligible for remote playback
2. Metadata about all the media resources may be sent to the remote playback device, so it can choose the remote playback source

Some more clarification could be done:

• Adding a step to 5.2.4 ("Establishing a connection with a remote playback device") to indicate when the selection of the remote playback source happens. (Instead, I updated a non-normative note at the end of these steps.)
• Clarifying what happens when remote playback is disconnected, i.e. does the user agent automatically revert to the .currentSrc that was playing before remote playback was initiated.

Let me know if you want further changes along these lines.

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[markafoltz]

PTAL @mounirlamouri @jernoble

[baylesj]

Not sure if the behavior on remote playback end is implementation specific or not, so if we expect that to be standardized (versus the implementation-specific playback source selection mechanism) I'd like to see that in this document.

I think the note for 5.2.4 is fine as is.

[jernoble]

LGTM.

[anssiko]

It seems @jernoble LGTM'd this PR conditional to @mounirlamouri's proposal #141 (comment) baked in?

@mfoltzgoogle feel free to update the PR per the proposal and try to get quick confirmation from @mounirlamouri and/or @baylesj and @jernoble prior to merge.

[markafoltz]

Updated the PR per the proposal from @mounirlamouri.

I also moved a separate note regarding source selection to this part of the spec, so that all the discussion of source selection is now in this section.

@mounirlamouri @baylesj @jernoble PTAL