[Sync] Use a proto to generate AutofillSyncStorageKey's storage keys.

components/autofill/core/browser/proto/autofill_sync.proto

+// Copyright 2016 The Chromium Authors. All rights reserved.

[maxbogue, 2017/01/04 01:42:45]

2017! @_@

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package autofill;
+
+// Used to convert between autofill::AutofillKey and a std::string that can be
+// given to Sync to uniquely identify an entity of ModelType syncer::AUTOFILL.

[pavely, 2017/01/04 18:09:56]

Consider "given to Sync" => "passed to sync as storage key".
I think mentioning "storage key" makes it clear which function will accept it.

[skym, 2017/01/05 01:31:51]

Done.

+message AutofillSyncStorageKey {
+  required string name = 1;
+  required string value = 2;
+}