[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.
+// 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
+// passed to sync as storage key to uniquely identify an entity of ModelType
+// syncer::AUTOFILL.
+message AutofillSyncStorageKey {
+  required string name = 1;

[Mathieu, 2017/01/06 01:51:29]

I don't particularly like "required" because I believe it crashes on
ParseFromString if not complete, but if handled with care I guess I'm OK as
there are other instances in Chrome

[skym, 2017/01/06 18:29:04]

I didn't realize that required was generally frowned upon, but the documentation
seems to have similar hesitations about required to you.
https://developers.google.com/protocol-buffers/docs/proto#specifying-field-rules

My thought process is that we're using the serialized to string bytes as our
key. We do not want these to vary. And I'm scared that has_x() returning
true/false will cause the serialized values to differ.

But I think required vs optional just kind of moves around where/how we break
when things go wrong. We don't get out of handling with care. So I'm thinking
I'll switch to optional and add some DCHECKs verifying that has_value() and
has_name() never return false when I add deserialization.

Let me know if you don't think this is reasonable.

+  required string value = 2;
+}