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+Private.h b/webrtc/api/objc/RTCSessionDescription+Private.h
index 2a9601d..9de8f0e 100644
--- a/webrtc/api/objc/RTCSessionDescription+Private.h
+++ b/webrtc/api/objc/RTCSessionDescription+Private.h
@@ -32,9 +32,9 @@
 - (instancetype)initWithNativeDescription:
     (const webrtc::SessionDescriptionInterface *)nativeDescription;
 
-+ (std::string)stringForType:(RTCSdpType)type;
++ (std::string)stdStringForType:(RTCSdpType)type;
 
-+ (RTCSdpType)typeForString:(const std::string &)string;
++ (RTCSdpType)typeForStdString:(const std::string &)string;
 
 @end