add mimetype definition from webrtc-stats

Author: fippo

index.bs

@@ -357,6 +357,14 @@ dictionary RTCEncodedVideoFrameMetadata {
             that is used to describe the format of the RTP payload.
         </p>
     </dd>
+    <dt>
+        <dfn>mimeType</dfn> of type <span class="idlMemberType">DOMString</span>
+    </dt>
+    <dd>
+        <p>
+            The codec MIME media type/subtype. e.g., video/vp8 or equivalent. 
+        </p>
+    </dd>
     <dt>
         <dfn>contributingSources</dfn> of type <span class=
             "idlMemberType">sequence&lt;unsigned long&gt;</span>