Simple Default ObjC video codec factories.

Move the simple video encoder/decoder factory from AppRTCMobile into the
public API so users who don't have special requirements for video codecs
can easily get started.

Also clean up the API a little.

This CL replaces the more flexible default factories in
https://webrtc-review.googlesource.com/c/src/+/7741 and clients that
want to implement their own codecs will have to supply their own
encoder/decoder factories as well. The benefits of the approach in
this CL are a simpler API and less effects on the rest of the code.

Bug: None
Change-Id: I4ed94090d778b4fc38b49864de1d4de4ff125d6a
Reviewed-on: https://webrtc-review.googlesource.com/15141
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20441}

sdk/objc/Framework/Headers/WebRTC/RTCVideoCodecFactory.h

@@ -33,4 +33,22 @@ RTC_EXPORT
 
 @end
 
+#pragma mark - Default factories
+
+/** These codec factories include support for all codecs bundled with WebRTC. If using custom
+ *  codecs, create custom implementations of RTCVideoEncoderFactory and RTCVideoDecoderFactory.
+ */
+RTC_EXPORT
+@interface RTCDefaultVideoEncoderFactory : NSObject <RTCVideoEncoderFactory>
+
+@property(nonatomic, retain) RTCVideoCodecInfo *preferredCodec;
+
++ (NSArray<RTCVideoCodecInfo *> *)supportedCodecs;
+
+@end
+
+RTC_EXPORT
+@interface RTCDefaultVideoDecoderFactory : NSObject <RTCVideoDecoderFactory>
+@end
+
 NS_ASSUME_NONNULL_END