[PeerConnection] Implement parameterless SetLocalDescription().

For background, motivation, requirements and implementation notes, see
https://docs.google.com/document/d/1XLwNN2kUIGGTwz9LQ0NwJNkcybi9oKnynUEZB1jGA14/edit?usp=sharing

The parameterless SetLocalDescription() will implicitly create an
offer or answer to be set by chaining create offer or answer with
setting the session description, as per spec:
https://w3c.github.io/webrtc-pc/#dom-peerconnection-setlocaldescription

Bug: chromium:980885
Change-Id: Ia430160869df18fd47b756b9adf9e7e23ba8e969
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157444
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29653}

api/peer_connection_proxy.h

@@ -100,6 +100,7 @@ PROXY_METHOD2(void,
               SetLocalDescription,
               SetSessionDescriptionObserver*,
               SessionDescriptionInterface*)
+PROXY_METHOD1(void, SetLocalDescription, SetSessionDescriptionObserver*)
 PROXY_METHOD2(void,
               SetRemoteDescription,
               SetSessionDescriptionObserver*,