Relax setServerCertificate() requirement (#575)

* Relax setServerCertificate() requirement

This change removed the requirement for key systems to support  requesting the server certificate via the "message" event, which is not supported by many real world key systems.

Author: xhwang-chromium

encrypted-media-respec.html

@@ -3116,15 +3116,15 @@ <h2>
               Provides a server certificate to be used to encrypt messages to the license server.
             </p>
             <p>
-              [=Key Systems=] that use such certificates MUST also support requesting the
-              certificate from the server via the [=Queue a "message" Event=] algorithm.
+              For [=Key Systems=] that use such certificates, applications SHOULD call
+              {{MediaKeys/setServerCertificate()}} to explicitly set the server certificate.
+              Otherwise, license exchange may fail.
             </p>
             <p class="note">
-              This method allows an application to proactively provide a server certificate to
-              implementations that support it to avoid the additional round trip should the [=CDM=]
-              request it. It is intended as an optimization, and applications are not required to
-              use it.
-            </p>
+              Some key systems also support requesting the certificate from the server via the
+              [=Queue a "message" Event=] algorithm. However, this is not supported by all key
+              systems that use such certificates.
+            </p>          
             <p>
               The server certificate contents are [=Key System=]-specific. It MUST NOT contain
               executable code.