Android: Replacement for JNIEnv::FindClass that works from any thread

This CL adds a replacement for JNIEnv::FindClass that works from any
thread, i.e. from native C++ threads as well. This function will be used
from the generated JNI code. Long term, we should stop using
classreferenceholder that relies on a hardcoded list of WebRTC classes.

Bug: webrtc:8278
Change-Id: I4f40c744325ac02b73bd8fa479ab50b684429dc2
Reviewed-on: https://webrtc-review.googlesource.com/20223
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20583}

sdk/android/src/java/org/webrtc/ClassLoader.java

@@ -0,0 +1,23 @@
+/*
+ *  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.
+ */
+
+package org.webrtc;
+
+/**
+ * This class provides a ClassLoader that is capable of loading WebRTC Java classes regardless of
+ * what thread it's called from. Such a ClassLoader is needed for the few cases where the JNI
+ * mechanism is unable to automatically determine the appropriate ClassLoader instance.
+ */
+class ClassLoader {
+  @CalledByNative
+  static Object getClassLoader() {
+    return ClassLoader.class.getClassLoader();
+  }
+}