Update type to match WebCodec VideoFrame.timestamp

Author: tonyherre

Diff for: index.bs

@@ -331,7 +331,7 @@ dictionary RTCEncodedVideoFrameMetadata {
     unsigned long synchronizationSource;
     octet payloadType;
     sequence<unsigned long> contributingSources;
-    DOMHighResTimeStamp captureTime;
+    long long captureTimestamp;    // microseconds
 };
 </pre>
 
@@ -367,18 +367,15 @@ dictionary RTCEncodedVideoFrameMetadata {
         </p>
     </dd>
     <dt>
-        <dfn>captureTime</dfn> of type <span class=
-            "idlMemberType">DOMHighResTimeStamp</span>
+        <dfn>captureTimestamp</dfn> of type <span class=
+            "idlMemberType">long long</span>
     </dt>
     <dd>
         <p>
-		For video frames coming from a local source, this is the time at which
-		the frame was captured by the camera.
-		For video frames coming from remote source, the capture time is based on the RTP
-		timestamp of the frame and estimated using clock synchronization.
-		This is best effort and can use methods like using RTCP SR as specified in
-		RFC 3550 Section 6.4.1, or by other alternative means if use by RTCP SR isn’t feasible.
-		As defined in VideoFrameCallbackMetadata.
+		For video frames coming from a local device, this is the timestamp in microseconds
+		at which the frame was captured by the camera.
+		This will have the same value as the basis for VideoFrameCallbackMetadata's captureTime
+		field.
         </p>
     </dd>
 </dl>