[Sync] Implements the loopback sync server.

components/sync/protocol/loopback_server.proto

Index: components/sync/protocol/loopback_server.proto
diff --git a/components/sync/protocol/loopback_server.proto b/components/sync/protocol/loopback_server.proto
new file mode 100644
index 0000000000000000000000000000000000000000..3f03524b9847979e3974231d84044236314756b3
--- /dev/null
+++ b/components/sync/protocol/loopback_server.proto
@@ -0,0 +1,62 @@
+// Copyright (c) 2012 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.
+//
+// Sync protocol for communication between sync client and server.

[pavely, 2016/10/06 23:48:46]

I don't think this comment applies here. I think this is protocol for persisting
loopback server state to disk.

[pastarmovj, 2016/10/13 14:13:40]

Done.

Also cleaned up some commented out fields.

+
+// Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
+// any fields in this file.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package sync_pb;
+
+import "sync.proto";
+
+// Serialization of the LoopbackServerEntity and its ancestors.
+message LoopbackServerEntity {
+  // Entity type mapping to one of the subclasses of LoopbackServerEntity.
+  enum Type {
+    BOOKMARK = 1;
+    PERMANENT = 2;
+    TOMBSTONE = 3;
+    UNIQUE = 4;
+  }
+
+  optional Type type = 1;
+  optional SyncEntity entity = 2;
+  // optional string id = 2;
+  optional int64 model_type = 3;
+  // optional int64 version = 4;
+  // optional string name = 5;
+  // optional EntitySpecifics specifics = 6;
+
+  // Type specific entries:
+  // optional string originator_cache_guid = 7;
+  // optional string originator_client_item_id = 8;
+  // optional UniquePosition unique_position = 9;
+  // optional bool is_folder = 10;
+  // optional string parent_id = 11;
+  // optional int64 creation_time = 12;
+  // optional int64 last_modified_time = 13;
+  // optional string server_defined_unique_tag = 14;
+  // optional string client_defined_unique_tag = 15;
+}
+
+// Contains the loopback server state.
+message LoopbackServerProto {
+  // The protocol buffer format version.
+  optional int64 version = 1;
+
+  optional int64 store_birthday = 2;
+
+  repeated LoopbackServerEntity entities = 3;
+
+  // All Keystore keys known to the server.
+  repeated string keystore_keys = 4;
+
+  // The last entity ID that was assigned to an entity.
+  optional int64 last_version_assigned = 5;
+}