Add a Clone() method to SessionDescriptionInterface

This should allow us to remove some SDP parsing in Chromium.

Bug: webrtc:12215
Change-Id: Ib85593d1c9226b29f2ec18617f945c76eca3b2d7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/197806
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32840}

api/jsep.h

@@ -136,6 +136,13 @@ class RTC_EXPORT SessionDescriptionInterface {
 
   virtual ~SessionDescriptionInterface() {}
 
+  // Create a new SessionDescriptionInterface object
+  // with the same values as the old object.
+  // TODO(bugs.webrtc.org:12215): Remove default implementation
+  virtual std::unique_ptr<SessionDescriptionInterface> Clone() {
+    return nullptr;
+  }
+
   // Only for use internally.
   virtual cricket::SessionDescription* description() = 0;
   virtual const cricket::SessionDescription* description() const = 0;