ipc: Move //crypto dependency into a Windows-only file.

ipc/brokerable_attachment_win.cc

+// 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.
+
+#include "ipc/brokerable_attachment.h"
+
+#include "crypto/random.h"
+
+namespace IPC {
+
+BrokerableAttachment::AttachmentId::AttachmentId() {
+  // In order to prevent mutually untrusted processes from stealing resources
+  // from one another, the nonce must be secret. This generates a 128-bit,
+  // cryptographicaly-strong random number.
+  crypto::RandBytes(nonce, BrokerableAttachment::kNonceSize);
+}
+
+}  // namespace IPC