[Sync] Add two more USS integration tests.

components/sync/engine_impl/model_type_worker.h

Index: components/sync/engine_impl/model_type_worker.h
diff --git a/components/sync/engine_impl/model_type_worker.h b/components/sync/engine_impl/model_type_worker.h
index d76e2769be1e202c0c470e67c19d27a8445021de..d5e05a817ec273116ca51f898d6d08f20af31832 100644
--- a/components/sync/engine_impl/model_type_worker.h
+++ b/components/sync/engine_impl/model_type_worker.h
@@ -162,7 +162,7 @@ class ModelTypeWorker : public syncer::UpdateHandler,
   // Interface used to access and send nudges to the sync scheduler. Not owned.
   syncer::NudgeHandler* nudge_handler_;
 
-  // A map of per-entity information known to this object.
+  // A map of per-entity information, keyed by client_tag_hash.
   //
   // When commits are pending, their information is stored here. This
   // information is dropped from memory when the commit succeeds or gets
@@ -170,9 +170,7 @@ class ModelTypeWorker : public syncer::UpdateHandler,
   //
   // This also stores some information related to received server state in
   // order to implement reflection blocking and conflict detection. This
-  // information is kept in memory indefinitely. With a bit more coordination
-  // with the model thread, we could optimize this to reduce memory usage in
-  // the steady state.
+  // information is kept in memory indefinitely.
   EntityMap entities_;
 
   // Accumulates all the updates from a single GetUpdates cycle in memory so