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

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

Index: components/autofill/core/browser/proto/autofill_sync.proto
diff --git a/components/autofill/core/browser/proto/autofill_sync.proto b/components/autofill/core/browser/proto/autofill_sync.proto
new file mode 100644
index 0000000000000000000000000000000000000000..0feb150949d68afa694edab9eb414231cd491966
--- /dev/null
+++ b/components/autofill/core/browser/proto/autofill_sync.proto
@@ -0,0 +1,16 @@
+// 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
+// given to Sync to uniquely identify an entity of ModelType syncer::AUTOFILL.
+message AutofillSyncStorageKey {
+  required string name = 1;
+  required string value = 2;
+}