Refactor some ObjC API init methods.

initWithFactory: is clumsy and makes classes difficult to mock out in
tests. By keeping methods on the factory, we can simply mock out the
factory's methods instead.

We can consider adding regular Obj-C like ctors if we move to making
the factory a singleton, but that requires further discussion.

BUG=
R=haysc@webrtc.org, hjon@webrtc.org

Review URL: https://codereview.webrtc.org/1820193002 .

Cr-Commit-Position: refs/heads/master@{#12089}
diff --git a/webrtc/api/objc/RTCAVFoundationVideoSource.h b/webrtc/api/objc/RTCAVFoundationVideoSource.h
index 1d1eac0..d7cdbef 100644
--- a/webrtc/api/objc/RTCAVFoundationVideoSource.h
+++ b/webrtc/api/objc/RTCAVFoundationVideoSource.h
@@ -25,8 +25,7 @@
  */
 @interface RTCAVFoundationVideoSource : RTCVideoSource
 
-- (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
-                    constraints:(nullable RTCMediaConstraints *)constraints;
+- (instancetype)init NS_UNAVAILABLE;
 
 /** Returns whether rear-facing camera is available for use. */
 @property(nonatomic, readonly) BOOL canUseBackCamera;