Changing RTCRtpContributingSource from an Interface to a dictionary

Relates to ORTC Issue 289: 
#289

ortc.html

@@ -1847,8 +1847,8 @@ <h3>dictionary RTCRtpContributingSource</h3>
                     If the RTP packet contains no CSRCs, then the <code><a>RTCRtpContributingSource</a></code> object corresponding to the
                     SSRC is updated, and the level value for the SSRC is updated based on the client-mixer header extension [[!RFC6464]] if present.
                 </p>
-                <dl class="idl" title="interface RTCRtpContributingSource">
-                    <dt>readonly attribute DOMHighResTimeStamp timestamp</dt>
+                <dl class="idl" title="dictionary RTCRtpContributingSource">
+                    <dt>DOMHighResTimeStamp timestamp</dt>
                     <dd>
                         <p>
                             The <var>timestamp</var> of type <code>DOMHighResTimeStamp</code>
@@ -1858,11 +1858,11 @@ <h3>dictionary RTCRtpContributingSource</h3>
                             a local clock.
                         </p>
                     </dd>
-                    <dt>readonly attribute unsigned long source</dt>
+                    <dt>unsigned long source</dt>
                     <dd>
                         <p>The CSRC or SSRC value of the contributing source.</p>
                     </dd>
-                    <dt>readonly attribute byte? audioLevel</dt>
+                    <dt>byte? audioLevel</dt>
                     <dd>
                         <p>
                             The audio level contained in the last RTP packet