Add base::UnguessableToken

mojo/common/common_custom_types.typemap

 # 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.
 
 mojom = "//mojo/common/common_custom_types.mojom"
 public_headers = [
   "//base/files/file_path.h",
+  "//base/nonce.h",
   "//base/strings/string16.h",
   "//base/time/time.h",
   "//base/values.h",
   "//base/version.h",
 ]
 traits_headers = [
   "//ipc/ipc_message_utils.h",
   "//mojo/common/common_custom_types_struct_traits.h",
 ]
 sources = [
   "//mojo/common:struct_traits",
 ]
 public_deps = [
   "//ipc",
 ]
 
 type_mappings = [
   "mojo.common.mojom.FilePath=base::FilePath",
   "mojo.common.mojom.DictionaryValue=base::DictionaryValue",
   "mojo.common.mojom.ListValue=base::ListValue",
+  "mojo.common.mojom.Nonce=base::Nonce",

[dcheng, 2016/09/13 23:19:06]

If we expect people to pass Nonce by value (and it sounds like we do?), then
this should be annotated with [copyable_pass_by_value]

[tguilbert, 2016/09/14 22:40:47]

From danakj@'s comment earlier in the thread, I think the preferred way is via
const ref.

   "mojo.common.mojom.String16=base::string16",
   "mojo.common.mojom.Time=base::Time[copyable_pass_by_value]",
   "mojo.common.mojom.TimeDelta=base::TimeDelta[copyable_pass_by_value]",
   "mojo.common.mojom.TimeTicks=base::TimeTicks[copyable_pass_by_value]",
   "mojo.common.mojom.Version=base::Version",
 ]