Update iOS AppRTCDemo to use level controller.

NOTRY=True
BUG=

Review-Url: https://codereview.webrtc.org/2294913003
Cr-Commit-Position: refs/heads/master@{#13977}
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCMediaConstraints.mm b/webrtc/sdk/objc/Framework/Classes/RTCMediaConstraints.mm
index f0f3c86..0a61f99 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCMediaConstraints.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCMediaConstraints.mm
@@ -30,6 +30,13 @@
     @(webrtc::MediaConstraintsInterface::kMinFrameRate);
 NSString * const kRTCMediaConstraintsMaxFrameRate =
     @(webrtc::MediaConstraintsInterface::kMaxFrameRate);
+NSString * const kRTCMediaConstraintsLevelControl =
+    @(webrtc::MediaConstraintsInterface::kLevelControl);
+
+NSString * const kRTCMediaConstraintsValueTrue =
+    @(webrtc::MediaConstraintsInterface::kValueTrue);
+NSString * const kRTCMediaConstraintsValueFalse =
+    @(webrtc::MediaConstraintsInterface::kValueFalse);
 
 namespace webrtc {