[Sync] Add ModelTypeStore implementation.

sync/BUILD.gn

Index: sync/BUILD.gn
diff --git a/sync/BUILD.gn b/sync/BUILD.gn
index 2307dbfa03ef44355e87ed84f581eea954b908f5..16ce82de95c10aaf1896884caf5e62d108a9cbac 100644
--- a/sync/BUILD.gn
+++ b/sync/BUILD.gn
@@ -167,6 +167,8 @@ source_set("sync_core") {
     "internal_api/js_sync_encryption_handler_observer.h",
     "internal_api/js_sync_manager_observer.cc",
     "internal_api/js_sync_manager_observer.h",
+    "internal_api/model_type_store_backend.cc",
+    "internal_api/model_type_store_impl.cc",
     "internal_api/protocol_event_buffer.cc",
     "internal_api/protocol_event_buffer.h",
     "internal_api/public/activation_context.h",
@@ -235,6 +237,8 @@ source_set("sync_core") {
     "internal_api/public/internal_components_factory_impl.h",
     "internal_api/public/model_type_processor.cc",
     "internal_api/public/model_type_processor.h",
+    "internal_api/public/model_type_store_backend.h",
+    "internal_api/public/model_type_store_impl.h",
     "internal_api/public/network_resources.h",
     "internal_api/public/non_blocking_sync_common.cc",
     "internal_api/public/non_blocking_sync_common.h",
@@ -637,6 +641,7 @@ test("sync_unit_tests") {
     "internal_api/js_mutation_event_observer_unittest.cc",
     "internal_api/js_sync_encryption_handler_observer_unittest.cc",
     "internal_api/js_sync_manager_observer_unittest.cc",
+    "internal_api/model_type_store_impl_unittest.cc",
     "internal_api/protocol_event_buffer_unittest.cc",
     "internal_api/public/base/attachment_id_proto_unittest.cc",
     "internal_api/public/base/cancelation_signal_unittest.cc",