Injectable logging

Allows passing a Loggable to PCFactory.initializationOptions, which
is then injected to Logging.java and logging.h. Future log messages
in both Java and native will then be passed to this Loggable.

Bug: webrtc:9225
Change-Id: I2ff693380639448301a78a93dc11d3a0106f0967
Reviewed-on: https://webrtc-review.googlesource.com/73243
Commit-Queue: Paulina Hensman <phensman@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23241}

rtc_base/java/src/org/webrtc/Loggable.java

@@ -0,0 +1,22 @@
+/*
+ *  Copyright (c) 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.
+ */
+
+package org.webrtc;
+
+import org.webrtc.Logging.Severity;
+
+/**
+ * Java interface for WebRTC logging. The default implementation uses webrtc.Logging.
+ *
+ * When injected, the Loggable will receive logging from both Java and native.
+ */
+public interface Loggable {
+  public void onLogMessage(String message, Severity severity, String tag);
+}