Rewrite WebRtcSession media tests as PeerConnection tests

Bug: webrtc:8222
Change-Id: I782a3227e30de70eb8f6c26a48723cb3510a84ad
Reviewed-on: https://webrtc-review.googlesource.com/6640
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20364}

pc/sdputils.h

@@ -23,6 +23,11 @@ namespace webrtc {
 std::unique_ptr<SessionDescriptionInterface> CloneSessionDescription(
     const SessionDescriptionInterface* sdesc);
 
+// Returns a copy of the given session description with the type changed.
+std::unique_ptr<SessionDescriptionInterface> CloneSessionDescriptionAsType(
+    const SessionDescriptionInterface* sdesc,
+    const std::string& type);
+
 // Function that takes a single session description content with its
 // corresponding transport and produces a boolean.
 typedef std::function<bool(const cricket::ContentInfo*,