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_tests.gyp b/webrtc/api/api_tests.gyp
index 83c83e1..466709c 100644
--- a/webrtc/api/api_tests.gyp
+++ b/webrtc/api/api_tests.gyp
@@ -21,6 +21,7 @@
           'sources': [
             'objctests/RTCIceCandidateTest.mm',
             'objctests/RTCIceServerTest.mm',
+            'objctests/RTCSessionDescriptionTest.mm',
           ],
           'xcode_settings': {
             'CLANG_ENABLE_OBJC_ARC': 'YES',