gin: Add out-of-line copy ctors for complex classes.

gin/object_template_builder.cc

Index: gin/object_template_builder.cc
diff --git a/gin/object_template_builder.cc b/gin/object_template_builder.cc
index 158131f206ce65718839f73096fe1549282d6064..a40df6037f39e4c5e95fc2c259567a0c96ce155e 100644
--- a/gin/object_template_builder.cc
+++ b/gin/object_template_builder.cc
@@ -145,6 +145,9 @@ ObjectTemplateBuilder::ObjectTemplateBuilder(v8::Isolate* isolate)
   template_->SetInternalFieldCount(kNumberOfInternalFields);
 }
 
+ObjectTemplateBuilder::ObjectTemplateBuilder(
+    const ObjectTemplateBuilder& other) = default;
+
 ObjectTemplateBuilder::~ObjectTemplateBuilder() {
 }