Tweaks for new Objective-C API.

BUG=

Review URL: https://codereview.webrtc.org/1696673003

Cr-Commit-Position: refs/heads/master@{#11872}
diff --git a/webrtc/api/objc/RTCSessionDescription.h b/webrtc/api/objc/RTCSessionDescription.h
index 5f00b1c..2635633 100644
--- a/webrtc/api/objc/RTCSessionDescription.h
+++ b/webrtc/api/objc/RTCSessionDescription.h
@@ -36,6 +36,10 @@
 - (instancetype)initWithType:(RTCSdpType)type sdp:(NSString *)sdp
     NS_DESIGNATED_INITIALIZER;
 
++ (NSString *)stringForType:(RTCSdpType)type;
+
++ (RTCSdpType)typeForString:(NSString *)string;
+
 @end
 
 NS_ASSUME_NONNULL_END