[EME] Requiring generateKeyRequest() in all cases for all key systems

https://www.w3.org/Bugs/Public/show_bug.cgi?id=16548

The proposal currently allows addKey() to be called without
calling generateKeyRequest(). This allows a very simple application as
shown in the example in section 7.1. However, such applications are
unlikely and supporting this adds complexity to implementations. Therefore,
I would like to always require generateKeyRequest(). Does anyone object?

The impact on the example in section 7.1 is shown in
http://dvcs.w3.org/hg/html-media/raw-file/tip/encrypted-media/encrypted-media.html#issue-disallow-addkey-only-example
.

Benefits include:
 * Simplifies user agent implementation - there is one API where a CDM can
be instantiated.
 * We can eliminate the keySystem parameter from addKey() and
clearKeyRequest()
 * Less confusion in the application and user agent related to the
potential for the keySystem value to change, such as in addKey() and
clearKeyRequest() calls.
 * It fits nicely with an object-oriented design, such as the one proposed
in bug 16613.