Update API for Objective-C RTCSessionDescription.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1524303006 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Commit-Position: refs/heads/master@{#11157}
diff --git a/webrtc/api/api.gyp b/webrtc/api/api.gyp
index d42d99d..ee04a4d 100644
--- a/webrtc/api/api.gyp
+++ b/webrtc/api/api.gyp
@@ -25,6 +25,9 @@
             'objc/RTCIceServer+Private.h',
             'objc/RTCIceServer.h',
             'objc/RTCIceServer.mm',
+            'objc/RTCSessionDescription+Private.h',
+            'objc/RTCSessionDescription.h',
+            'objc/RTCSessionDescription.mm',
           ],
           'xcode_settings': {
             'CLANG_ENABLE_OBJC_ARC': 'YES',