Add new Logger to iOS SDK to be able to intercept logs and report them to the appropriate system

Bug: webrtc:8695
Change-Id: I4f81529222dcaf3dded1a0ab93fabf5cda1fdef8
Reviewed-on: https://webrtc-review.googlesource.com/37001
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21821}

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

@@ -0,0 +1,35 @@
+/*
+ *  Copyright 2018 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 <Foundation/Foundation.h>
+
+#import <WebRTC/RTCLogging.h>
+#import <WebRTC/RTCMacros.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+// This class intercepts WebRTC logs and forwards them to a registered block.
+// This class is not threadsafe.
+RTC_EXPORT
+@interface RTCCallbackLogger : NSObject
+
+// The severity level to capture. The default is kRTCLoggingSeverityInfo.
+@property(nonatomic, assign) RTCLoggingSeverity severity;
+
+// The callback will be called on the same thread that does the logging, so
+// if the logging callback can be slow it may be a good idea to implement
+// dispatching to some other queue.
+- (void)start:(nullable void (^)(NSString*))callback;
+
+- (void)stop;
+
+@end
+
+NS_ASSUME_NONNULL_END