Tweaks for new Objective-C API.

BUG=

Review URL: https://codereview.webrtc.org/1696673003

Cr-Commit-Position: refs/heads/master@{#11872}
diff --git a/webrtc/api/objc/RTCAVFoundationVideoSource.h b/webrtc/api/objc/RTCAVFoundationVideoSource.h
index 1fa19c5..e340444 100644
--- a/webrtc/api/objc/RTCAVFoundationVideoSource.h
+++ b/webrtc/api/objc/RTCAVFoundationVideoSource.h
@@ -26,7 +26,7 @@
 @interface RTCAVFoundationVideoSource : RTCVideoSource
 
 - (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
-                    constraints:(RTCMediaConstraints *)constraints;
+                    constraints:(nullable RTCMediaConstraints *)constraints;
 
 /** Switches the camera being used (either front or back). */
 @property(nonatomic, assign) BOOL useBackCamera;