Compile rtc_api_objc on Mac.

BUG=

Review URL: https://codereview.webrtc.org/1726213002

Cr-Commit-Position: refs/heads/master@{#11771}
diff --git a/webrtc/api/objctests/RTCMediaConstraintsTest.mm b/webrtc/api/objctests/RTCMediaConstraintsTest.mm
index 44ffe3d..f73a6ca 100644
--- a/webrtc/api/objctests/RTCMediaConstraintsTest.mm
+++ b/webrtc/api/objctests/RTCMediaConstraintsTest.mm
@@ -47,7 +47,7 @@
   EXPECT_EQ(constraints.count, nativeConstraints.size());
 
   for (NSString *key in constraints) {
-    NSString *value = constraints[key];
+    NSString *value = [constraints objectForKey:key];
 
     std::string nativeValue;
     bool found = nativeConstraints.FindFirst(key.stdString, &nativeValue);