Provide dummy Android UI thread with looper for C++ unit tests

base/test/android/java/src/org/chromium/base/TestUiThread.java

Index: base/test/android/java/src/org/chromium/base/TestUiThread.java
diff --git a/base/test/android/java/src/org/chromium/base/TestUiThread.java b/base/test/android/java/src/org/chromium/base/TestUiThread.java
new file mode 100644
index 0000000000000000000000000000000000000000..77f96606ad6dbddec88ef9e46cda7996e7934f2e
--- /dev/null
+++ b/base/test/android/java/src/org/chromium/base/TestUiThread.java
@@ -0,0 +1,49 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.base;
+
+import android.os.Looper;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * Set up a thread as the Chromium UI Thread, and run its looper. This is is intended for C++ unit
+ * tests (e.g. the net unit tests) that don't run with the UI thread as their main looper, but test
+ * code that, on Android, uses UI thread events, so need a running UI thread.
+ */
+@ThreadSafe
+public class TestUiThread {
+    private static final AtomicBoolean sStarted = new AtomicBoolean(false);
+    private static final String TAG = Log.makeTag("TestUiThread");
+
+    @CalledByNative
+    private static void loop() {
+        // @{link ThreadUtils#setUiThread(Looper)} can only be called once in a test run, so do this
+        // once, and leave it running.
+        if (sStarted.getAndSet(true)) return;
+
+        final CountDownLatch startLatch = new CountDownLatch(1);
+        new Thread(new Runnable() {
+
+            @Override
+            public void run() {
+                Looper.prepare();
+                ThreadUtils.setUiThread(Looper.myLooper());
+                startLatch.countDown();
+                Looper.loop();
+            }
+
+        }).start();
+
+        try {
+            startLatch.await();
+        } catch (InterruptedException e) {
+            Log.e(TAG, "Failed to set UI Thread");
+        }
+    }
+}