Add HttpUrlConnection backed implementation of CronetEngine.

components/cronet/android/api/src/org/chromium/net/Preconditions.java

+// 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.net;
+
+import java.nio.ByteBuffer;
+
+final class Preconditions {
+    private Preconditions() {}
+
+    static void checkDirect(ByteBuffer buffer) {
+        if (!buffer.isDirect()) {
+            throw new IllegalArgumentException("byteBuffer must be a direct ByteBuffer.");
+        }
+    }
+
+    static void checkHasRemaining(ByteBuffer buffer) {
+        if (!buffer.hasRemaining()) {
+            throw new IllegalArgumentException("ByteBuffer is already full.");
+        }
+    }
+}