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/peerconnectionproxy.h b/webrtc/api/peerconnectionproxy.h
index f002014..2110ee2 100644
--- a/webrtc/api/peerconnectionproxy.h
+++ b/webrtc/api/peerconnectionproxy.h
@@ -47,6 +47,14 @@
                 CreateDataChannel, const std::string&, const DataChannelInit*)
   PROXY_CONSTMETHOD0(const SessionDescriptionInterface*, local_description)
   PROXY_CONSTMETHOD0(const SessionDescriptionInterface*, remote_description)
+  PROXY_CONSTMETHOD0(const SessionDescriptionInterface*,
+                     pending_local_description)
+  PROXY_CONSTMETHOD0(const SessionDescriptionInterface*,
+                     pending_remote_description)
+  PROXY_CONSTMETHOD0(const SessionDescriptionInterface*,
+                     current_local_description)
+  PROXY_CONSTMETHOD0(const SessionDescriptionInterface*,
+                     current_remote_description)
   PROXY_METHOD2(void, CreateOffer, CreateSessionDescriptionObserver*,
                 const MediaConstraintsInterface*)
   PROXY_METHOD2(void, CreateAnswer, CreateSessionDescriptionObserver*,