ObjC: Deprecate codec settings parameter in startDecode method.

This parameter is being removed from the C++ API, remove it from the
ObjC API also. It was never used for anything by the H264 decoder.

Bug: webrtc:9107
Change-Id: I5222eac932a4e7d4129d803f8126b5e8d0b027b6
Reviewed-on: https://webrtc-review.googlesource.com/66740
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22730}

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

@@ -166,7 +166,8 @@ RTC_EXPORT
 
 - (void)setCallback:(RTCVideoDecoderCallback)callback;
 - (NSInteger)startDecodeWithSettings:(RTCVideoEncoderSettings *)settings
-                       numberOfCores:(int)numberOfCores;
+                       numberOfCores:(int)numberOfCores
+    DEPRECATED_MSG_ATTRIBUTE("use startDecodeWithNumberOfCores: instead");
 - (NSInteger)releaseDecoder;
 - (NSInteger)decode:(RTCEncodedImage *)encodedImage
           missingFrames:(BOOL)missingFrames
@@ -175,6 +176,10 @@ RTC_EXPORT
            renderTimeMs:(int64_t)renderTimeMs;
 - (NSString *)implementationName;
 
+// TODO(andersc): Make non-optional when `startDecodeWithSettings:numberOfCores:` is removed.
+@optional
+- (NSInteger)startDecodeWithNumberOfCores:(int)numberOfCores;
+
 @end
 
 NS_ASSUME_NONNULL_END