Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCMediaConstraints.mm

Index: webrtc/api/objc/RTCMediaConstraints.mm
diff --git a/webrtc/api/objc/RTCMediaConstraints.mm b/webrtc/api/objc/RTCMediaConstraints.mm
index a53a517747f7a2ebcf9fb8f71b36a7ae8f3379bd..a13b4fec5aabc5da03255ba0b9352ae492121255 100644
--- a/webrtc/api/objc/RTCMediaConstraints.mm
+++ b/webrtc/api/objc/RTCMediaConstraints.mm
@@ -38,14 +38,14 @@ MediaConstraints::GetOptional() const {
 
 
 @implementation RTCMediaConstraints {
-  NSDictionary<NSString *, NSString *> *_mandatory;
-  NSDictionary<NSString *, NSString *> *_optional;
+  NSDictionary/*<NSString *, NSString *>*/ *_mandatory;
+  NSDictionary/*<NSString *, NSString *>*/ *_optional;
 }
 
 - (instancetype)initWithMandatoryConstraints:
-    (NSDictionary<NSString *, NSString *> *)mandatory
+    (NSDictionary/*<NSString *, NSString *>*/ *)mandatory
                          optionalConstraints:
-    (NSDictionary<NSString *, NSString *> *)optional {
+    (NSDictionary/*<NSString *, NSString *>*/ *)optional {
   if (self = [super init]) {
     _mandatory = [[NSDictionary alloc] initWithDictionary:mandatory
                                                 copyItems:YES];
@@ -76,15 +76,16 @@ MediaConstraints::GetOptional() const {
 
 + (webrtc::MediaConstraintsInterface::Constraints)
     nativeConstraintsForConstraints:
-        (NSDictionary<NSString *, NSString *> *)constraints {
+        (NSDictionary/*<NSString *, NSString *>*/ *)constraints {
   webrtc::MediaConstraintsInterface::Constraints nativeConstraints;
   for (NSString *key in constraints) {
     NSAssert([key isKindOfClass:[NSString class]],
              @"%@ is not an NSString.", key);
-    NSAssert([constraints[key] isKindOfClass:[NSString class]],
-             @"%@ is not an NSString.", constraints[key]);
+    NSString *value = [constraints objectForKey:key];
+    NSAssert([value isKindOfClass:[NSString class]],
+             @"%@ is not an NSString.", value);
     nativeConstraints.push_back(webrtc::MediaConstraintsInterface::Constraint(
-        key.stdString, constraints[key].stdString));
+        key.stdString, value.stdString));
   }
   return nativeConstraints;
 }