[Sync] Add plumbing of context from client to server

sync/engine/directory_update_handler.cc

Index: sync/engine/directory_update_handler.cc
diff --git a/sync/engine/directory_update_handler.cc b/sync/engine/directory_update_handler.cc
index 9d97765195fc1064861d9cef1064b90d276b2a6d..55095202fa800d363e1b9d2fd931f10df3273007 100644
--- a/sync/engine/directory_update_handler.cc
+++ b/sync/engine/directory_update_handler.cc
@@ -31,8 +31,15 @@ void DirectoryUpdateHandler::GetDownloadProgress(
   dir_->GetDownloadProgress(type_, progress_marker);
 }
 
+void DirectoryUpdateHandler::GetDataTypeContext(
+    sync_pb::DataTypeContext* context) const {
+  syncable::ModelNeutralWriteTransaction trans(FROM_HERE, SYNCER, dir_);
+  dir_->GetDataTypeContext(&trans, type_, context);
+}
+
 void DirectoryUpdateHandler::ProcessGetUpdatesResponse(
     const sync_pb::DataTypeProgressMarker& progress_marker,
+    const sync_pb::DataTypeContext& mutated_context,
     const SyncEntityList& applicable_updates,
     sessions::StatusController* status) {
   syncable::ModelNeutralWriteTransaction trans(FROM_HERE, SYNCER, dir_);
@@ -42,6 +49,20 @@ void DirectoryUpdateHandler::ProcessGetUpdatesResponse(
     ExpireEntriesIfNeeded(&trans, progress_marker);
     UpdateProgressMarker(progress_marker);
   }
+
+  if (mutated_context.has_context()) {
+    sync_pb::DataTypeContext local_context;
+    dir_->GetDataTypeContext(&trans, type_, &local_context);
+
+    // Only update the local context if it is still relevant. If the local
+    // version is higher, it means a local change happened while the mutation
+    // was in flight, and the local context takes priority.
+    if (mutated_context.version() >= local_context.version() &&
+        local_context.context() != mutated_context.context()) {
+      dir_->SetDataTypeContext(&trans, type_, mutated_context);
+      // TODO(zea): trigger the datatype's UpdateDataTypeContext method.
+    }
+  }
 }
 
 void DirectoryUpdateHandler::ApplyUpdates(sessions::StatusController* status) {