Implement current/pending session description methods.

BUG=webrtc:6917

Review-Url: https://codereview.webrtc.org/2590753002
Cr-Commit-Position: refs/heads/master@{#15722}
diff --git a/webrtc/api/peerconnection.h b/webrtc/api/peerconnection.h
index 70fd867..5269e3a 100644
--- a/webrtc/api/peerconnection.h
+++ b/webrtc/api/peerconnection.h
@@ -113,6 +113,12 @@
 
   const SessionDescriptionInterface* local_description() const override;
   const SessionDescriptionInterface* remote_description() const override;
+  const SessionDescriptionInterface* current_local_description() const override;
+  const SessionDescriptionInterface* current_remote_description()
+      const override;
+  const SessionDescriptionInterface* pending_local_description() const override;
+  const SessionDescriptionInterface* pending_remote_description()
+      const override;
 
   // JSEP01
   // Deprecated, use version without constraints.