[EME] API changes: requestMediaKeySystemAccess() and MediaKeyStatuses

Below are summaries of two recent significant API changes.

Application developers, please review the usefulness and usability of these
features as specified. If you have comments or concerns, please start a
thread or open an issue <https://github.com/w3c/encrypted-media/issues/new>
on GitHub.


*requestMediaKeySystemAccess() and MediaKeySystemAccess (bug 25923
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=25923>)*
requestMediaKeySystemAccess() does feature detection - replacing
isTypeSupported() - and provides a MediaKeySystemAccess, which is used to
create a MediaKeys object.


   - requestMediaKeySystemAccess() and feature detection:
   https://w3c.github.io/encrypted-media/#obtaining-access-to-key-systems
   - MediaKeySystemAccess:
   https://w3c.github.io/encrypted-media/#mediakeysystemaccess-interface`
   - Example usage:
   https://w3c.github.io/encrypted-media/#example-selecting-key-system


Note: There have been significant changes to the APIs and algorithms
since requestMediaKeySystemAccess() was first implemented. See the comments
starting with comment 44
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=25923#c44> for details on
these changes.


*Replace getUsableKeyIds() with a map-like keyStatuses attribute (bug 27111
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=27111>)*

   - Map-like MediaKeyStatuses interface:
   https://w3c.github.io/encrypted-media/#mediakeystatuses-interface
   - Initial status values:
   https://w3c.github.io/encrypted-media/#idl-def-MediaKeyStatus
   - keyStatuses attribute of MediaKeySession, including a note about how
   it may be updated:
   https://w3c.github.io/encrypted-media/#widl-MediaKeySession-keyStatuses
   - Example usage:
   https://w3c.github.io/encrypted-media/#example-using-all-events