[Sync] Implement support for updating storage key for new entities

components/sync/model_impl/processor_entity_tracker.cc

Index: components/sync/model_impl/processor_entity_tracker.cc
diff --git a/components/sync/model_impl/processor_entity_tracker.cc b/components/sync/model_impl/processor_entity_tracker.cc
index 7d0b6b331982e95c17beaa33630fefb2932e3165..f6fe203d2d3dff93bd987e064d26ccea71aa91b3 100644
--- a/components/sync/model_impl/processor_entity_tracker.cc
+++ b/components/sync/model_impl/processor_entity_tracker.cc
@@ -45,6 +45,7 @@ std::unique_ptr<ProcessorEntityTracker> ProcessorEntityTracker::CreateNew(
 std::unique_ptr<ProcessorEntityTracker>
 ProcessorEntityTracker::CreateFromMetadata(const std::string& storage_key,
                                            sync_pb::EntityMetadata* metadata) {
+  DCHECK(!storage_key.empty());
   return std::unique_ptr<ProcessorEntityTracker>(
       new ProcessorEntityTracker(storage_key, metadata));
 }
@@ -61,8 +62,10 @@ ProcessorEntityTracker::ProcessorEntityTracker(
 
 ProcessorEntityTracker::~ProcessorEntityTracker() {}
 
-void ProcessorEntityTracker::SetStorageKey(const std::string& new_key) {
-  storage_key_ = new_key;
+void ProcessorEntityTracker::SetStorageKey(const std::string& storage_key) {
+  DCHECK(storage_key_.empty());
+  DCHECK(!storage_key.empty());
+  storage_key_ = storage_key;
 }
 
 void ProcessorEntityTracker::SetCommitData(EntityData* data) {