Add interfaces for Java TaskRunners with a subset of the C++ functionality

The intent of this patch is to define the interfaces which will be implemented
by subsequent patches.  The interfaces are a subset of the C++ ones including
what we believe is the minimum requried to be useful.

Design doc: https://docs.google.com/document/d/1z1BDq9vzcEpkhN9LSPF5XMnZ0kLJ8mWWkNAi4OI7cos/edit#

Bug: 863341, 872372
Change-Id: Ic608660a13c55c4b17d3ba5fa3bb636783735645
Reviewed-on: https://chromium-review.googlesource.com/1245790
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595057}

base/BUILD.gn

@@ -2973,7 +2973,10 @@ if (is_android) {
       "android/java/src/org/chromium/base/memory/MemoryPressureUma.java",
       "android/java/src/org/chromium/base/task/AsyncTask.java",
       "android/java/src/org/chromium/base/task/ChromeThreadPoolExecutor.java",
+      "android/java/src/org/chromium/base/task/SequencedTaskRunner.java",
       "android/java/src/org/chromium/base/task/SerialExecutor.java",
+      "android/java/src/org/chromium/base/task/SingleThreadTaskRunner.java",
+      "android/java/src/org/chromium/base/task/TaskRunner.java",
     ]
 
     # New versions of BuildConfig.java and NativeLibraries.java

base/android/java/src/org/chromium/base/task/SequencedTaskRunner.java

+// Copyright 2018 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.task;
+
+/**
+ * Tasks posted will be run in order with respect to this sequence, but they may be executed
+ * on arbitrary threads. Unless specified otherwise by the provider of a given
+ * SequencedTaskRunner, tasks posted to it have no ordering, nor mutual exclusion, execution
+ * guarantees w.r.t. other SequencedTaskRunners.
+ */
+public interface SequencedTaskRunner extends TaskRunner {}

base/android/java/src/org/chromium/base/task/SingleThreadTaskRunner.java

+// Copyright 2018 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.task;
+
+/**
+ * Tasks posted will be run in order on a single thread.
+ */
+public interface SingleThreadTaskRunner extends SequencedTaskRunner {}

base/android/java/src/org/chromium/base/task/TaskRunner.java

+// Copyright 2018 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.task;
+
+/**
+ * A task queue that posts Java tasks onto the C++ browser scheduler, if loaded. Otherwise this
+ * will be backed by an {@link android.os.Handler} or the java thread pool. The TaskQueue interface
+ * provides no guarantee over the order or the thread on which the task will be executed.
+ *
+ * Very similar to {@link java.util.concurrent.Executor} but conforms to chromium terminology.
+ */
+public interface TaskRunner {
+    /**
+     * Posts a task to run immediately.
+     *
+     * @param task The task to be run immediately.
+     */
+    public void postTask(Runnable task);
+
+    /**
+     * Instructs the TaskRunner to initialize the native TaskRunner and migrate any tasks over to
+     * it.
+     */
+    void initNativeTaskRunner();
+}