[Sync] Add a sanity integration test for USS.

components/sync/test/fake_server/tombstone_entity.h

Index: components/sync/test/fake_server/tombstone_entity.h
diff --git a/components/sync/test/fake_server/tombstone_entity.h b/components/sync/test/fake_server/tombstone_entity.h
index fa99e5d24a931b1add2fcccb872f5795ca495ebd..656d05dab5296b560801d2b4335f37b966ae35cd 100644
--- a/components/sync/test/fake_server/tombstone_entity.h
+++ b/components/sync/test/fake_server/tombstone_entity.h
@@ -19,7 +19,9 @@ class TombstoneEntity : public FakeServerEntity {
   ~TombstoneEntity() override;
 
   // Factory function for TombstoneEntity.
-  static std::unique_ptr<FakeServerEntity> Create(const std::string& id);
+  static std::unique_ptr<FakeServerEntity> Create(
+      const std::string& id,
+      const std::string& client_defined_unique_tag);
 
   // FakeServerEntity implementation.
   bool RequiresParentId() const override;
@@ -28,7 +30,9 @@ class TombstoneEntity : public FakeServerEntity {
   bool IsDeleted() const override;
 
  private:
-  TombstoneEntity(const std::string& id, const syncer::ModelType& model_type);
+  TombstoneEntity(const std::string& id,
+                  const std::string& client_defined_unique_tag,
+                  const syncer::ModelType& model_type);
 };
 
 }  // namespace fake_server