Don't call virtual methods in TypeNamespace constructors

While probably safe in this particular context because of this specific
class structure, it is bad practice to call virtual methods in constructors.

Bug: None
Test: unittests continue to pass

Change-Id: I3c9a435efd89a2c2bd7ce576aed585b8ab766bda
diff --git a/ast_java_unittest.cpp b/ast_java_unittest.cpp
index 7c09684..3981363 100644
--- a/ast_java_unittest.cpp
+++ b/ast_java_unittest.cpp
@@ -38,6 +38,7 @@
 
 TEST(AstJavaTests, GeneratesClass) {
   JavaTypeNamespace types;
+  types.Init();
   Type class_type(&types, "TestClass", ValidatableType::KIND_GENERATED,
                   false, false);
   Type extend_type(&types, "SuperClass", ValidatableType::KIND_BUILT_IN,