Convert NaCl renderer-loader messages to mojo.

components/nacl/common/nacl.mojom

+// Copyright 2016 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.
+
+module nacl.mojom;
+
+[Native]
+enum NaClErrorCode;
+
+interface NaClRendererHost {
+  // This message must be synchronous to ensure that the exit status is sent
+  // from NaCl to the renderer before the NaCl process exits very soon after.
+  [Sync]
+  ReportExitStatus(int32 exit_status) => ();
+
+  // This message must be synchronous to ensure that the load status is sent
+  // from NaCl to the renderer before the NaCl process exits very soon after.
+  [Sync]
+  ReportLoadStatus(NaClErrorCode load_status) => ();
+
+  ProvideExitControl(NaClExitControl exit_control);
+};
+
+// An empty interface whose closure indicates that the NaCl plugin should exit.

[Mark Seaborn, 2016/12/19 19:00:36]

Nit: "plugin" -> "loader process"

In the NaCl/Chromium code, we need to distinguish between the part of the plugin
that runs in the renderer process and the part that runs in the NaCl loader
process.  (The latter is not a great term, but it's the term that's used in the
codebase.)

Also, rather than saying "closure", how about "When this is closed, it indicates
that...".  The term "closure" could be misinterpreted.

[Sam McNally, 2016/12/19 23:12:12]

Done.

+interface NaClExitControl {};