Adding constructor to StreamSelector.

This allows us to update downstreams to no longer use the constructor
that includes groupid. This is part of a small cleanup of StreamParams.

Bug: webrtc:9042
Change-Id: I0343fd9213157614023e5990f6e776b4f56de144
Reviewed-on: https://webrtc-review.googlesource.com/63421
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22695}

media/base/streamparams.h

@@ -188,6 +188,9 @@ struct StreamSelector {
       streamid(streamid) {
   }
 
+  explicit StreamSelector(const std::string& streamid)
+      : ssrc(0), streamid(streamid) {}
+
   bool Matches(const StreamParams& stream) const {
     if (ssrc == 0) {
       return stream.groupid == groupid && stream.id == streamid;