Add AsyncWaiter implementation to java API.

mojo/public/java/src/org/chromium/mojo/system/AsyncWaiter.java

+// Copyright 2014 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.mojo.system;
+
+import org.chromium.mojo.system.Core.WaitFlags;
+
+/**
+ * Help classes allowing asynchronously calling wait on a background thread, and pass the result to

[bulach, 2014/05/15 19:32:58]

nit: the callback has also an error condition, so perhaps:

Allows asynchronously waiting on a background thread.
The given callback {@link AsyncWaiterCallback} will be notified
on the current thread.
Returns a {@link Cancellable} object that can be used to stop waiting
(and must only be used on the current thread.). The cancellable becomes
invalid once the callback has been notified.

[rmcilroy, 2014/05/15 22:36:51]

Most of this is describes the asyncWait method.  I would simply have here
something like:

A class which implements the AsyncWaiter allows asynchronously waiting on a 
background thread.  

Then augment the asyncWait javadoc as below.

[qsr, 2014/05/19 12:12:09]

Done.

+ * the given {@link AsyncWaiterCallback} on the current thread. Returns a {@link Cancellable} object
+ * that can be used to stop waiting. This object becomes invalid once the callback runs.
+ */
+public interface AsyncWaiter {
+
+    /**
+     * Allow to stop waiting.

[rmcilroy, 2014/05/15 22:36:51]

Allows cancelation of an asyncWait operation.

[qsr, 2014/05/19 12:12:09]

Done.

+     */
+    interface Cancellable {
+        /**
+         * Stop waiting. Has no effect if the callback has already been called. Must be called from

[rmcilroy, 2014/05/15 22:36:51]

Cancels an asyncWait operation.  Has no effect if the operation has already been
canceled or the callback has already been called. Must be called from the same
thread as {@link AsyncWaiter#asyncWait(Handle, WaitFlags, long,
AsyncWaiterCallback)} was called from.

[qsr, 2014/05/19 12:12:09]

Done.

+         * the same thread {@link AsyncWaiter#asyncWait(Handle, WaitFlags, long,
+         * AsyncWaiterCallback)} has been called.
+         */
+        void cancel();
+    }
+
+    /**
+     * Asynchronously call wait on a background thread, and pass the result of wait to the given
+     * callback on the current thread.

[rmcilroy, 2014/05/15 22:36:51]

Asynchronously call wait on a background thread.  The given
{@link AsyncWaiterCallback} will be notified of the result of the wait 
on the same thread as asyncWait was called.

@return a {@link Cancellable} object that can be used to cancel waiting.
The cancellable should only be used on the current thread, and becomes
invalid once the callback has been notified.

[qsr, 2014/05/19 12:12:09]

Done.

+     */
+    Cancellable asyncWait(Handle handle,
+            WaitFlags flags, long deadline,
+            AsyncWaiterCallback callback);
+
+}