I have created a PR for RtpSender.getCapabilities and RtpReceiver.getCapabilities

Here's the PR:

https://github.com/w3c/webrtc-pc/pull/255

Here's the diff:

+        <dl class="idl" title="dictionary RTCRtpCapabilities">
+          <dt>sequence&lt;RTCRtpCodecCapability&gt; codecs</dt>
+          <dd><p>Supported codecs</p></dd>
+
+          <dt>sequence&lt;RTCRtpHeaderExtensionCapability&gt;
headerExtensions</dt>
+          <dd><p>Supported RTP header extensions.</p></dd>
+        </dl>
+
+        <dl class="idl" title="dictionary RTCRtpCodecCapability">
+          <dt>DOMString name</dt>
+          <dd><p>The codec MIME type. Valid types are listed in
[IANA-RTP-2].</p></dd>
+        </dl>
+
+        <dl class="idl" title="dictionary RTCRtpHeaderExtensionCapability">
+          <dt>DOMString uri</dt>
+          <dd><p>The URI of the RTP header extension, as defined in
[RFC5285]</p></dd>
+        </dl>

+          <dt>static RTCRtpCapabilities getCapabilities(DOMString
kind)</dt>
+          <dd>
+            <p>The <dfn
id="dom-rtpsender-getcapabilities"><code>RTCRtpSender.getCapabilities</code></dfn>
+            method returns the most optimist view on the capabilities
+            of the system for sending media of the given kind. It does
+            not reserve any resources, ports, or other state but is
+            meant to provide a way to discover the types of
+            capabilities of the browser including which codecs may be
+            supported.
+            </p>
+          </dd>
+          </dt>

+          <dt>static RTCRtpCapabilities getCapabilities(DOMString
kind)</dt>
+          <dd>
+            <p>The <dfn
id="dom-rtpreceiver-getcapabilities"><code>RTCRtpSender.getCapabilities</code></dfn>
+            method returns the most optimist view on the capabilities
+            of the system for receiving media of the given kind. It
+            does not reserve any resources, ports, or other state but
+            is meant to provide a way to discover the types of
+            capabilities of the browser including which codecs may be
+            supported.
+            </p>
+          </dd>
+          </dt>


Questions for the WG:

1.  Is there enough consensus from the WG to add this?  I'm in favor of it
:).

2.  Should getCapabilities return a Promise?  I think it doesn't need to
be, so I made it not return a Promise.

3.  Should codec.name be a MIME type ("video/vp8") or just a name ("vp8")?
If a MIME type, should it be code.mimeType instead of codec.name?  Or
should we have both codec.name and codec.mimeType.   I've made it codec.name
with MIME type here, which matches ORTC.  But I'm currently thinking I'd
prefer codec.name with a name instead.  I'd also be happy with having both
codec.name and codec.mimeType.