Reland "Simple Default ObjC video codec factories."

This is a reland of 30915a742d86df55ac5c04501c0e8104675a612e
Original change's description:
> 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}

Bug: None
Change-Id: If0910cc540dc835dfec4eeb5bea527d88482d110
Reviewed-on: https://webrtc-review.googlesource.com/16780
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20476}

sdk/objc/Framework/Classes/PeerConnection/RTCDefaultVideoDecoderFactory.m

@@ -0,0 +1,39 @@
+/*
+ *  Copyright 2017 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#import "WebRTC/RTCVideoCodecFactory.h"
+
+#import "WebRTC/RTCVideoCodecH264.h"
+#import "WebRTC/RTCVideoDecoderVP8.h"
+#import "WebRTC/RTCVideoDecoderVP9.h"
+
+@implementation RTCDefaultVideoDecoderFactory
+
+- (id<RTCVideoDecoder>)createDecoder:(RTCVideoCodecInfo *)info {
+  if ([info.name isEqualToString:kRTCVideoCodecH264Name]) {
+    return [[RTCVideoDecoderH264 alloc] init];
+  } else if ([info.name isEqualToString:kRTCVideoCodecVp8Name]) {
+    return [RTCVideoDecoderVP8 vp8Decoder];
+  } else if ([info.name isEqualToString:kRTCVideoCodecVp9Name]) {
+    return [RTCVideoDecoderVP9 vp9Decoder];
+  }
+
+  return nil;
+}
+
+- (NSArray<RTCVideoCodecInfo *> *)supportedCodecs {
+  return @[
+    [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecH264Name],
+    [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp8Name],
+    [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp9Name]
+  ];
+}
+
+@end