MediaSessionActionHandler callback's type is never correctly chosen

[marcoscaceres]

The spec says:

<li>
  Run <var>handler</var> with the <var>details</var> parameter set to:
  {{MediaSessionActionDetails}}.
</li>

But the actual dictionary type is never selected. It needs to match the action, and that dictionary should be properly initialized.

[marcoscaceres]

The problem is that it's not wired up correctly.

This:

The MediaSessionActionHandler MUST be run with the details parameter whose dictionary type is:

Should be moved to the where the actual callback handler is called. This should really be a switch statement based on the action type.

[marcoscaceres]

Further, for example, MediaSessionSeekToActionDetails:

dictionary MediaSessionSeekToActionDetails : MediaSessionActionDetails {   
  required double seekTime;   
   boolean fastSeek; 
};

The spec never says what to set .seekTime to, or what .fastSeek should be set to. It just provides a description.