Fix nits in CXFA_Document::DataMerge_CopyContainer().

- Sort the cases in the switch statement.
- Assert the node is a container. It is not obvious what the cases
  represent.
- Return from the switch statement instead of breaking out.

Change-Id: I25a1098f757118b436912a85a41e0dd6c3bea869
Reviewed-on: https://pdfium-review.googlesource.com/c/50992
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/xfa/fxfa/parser/cxfa_document.cpp b/xfa/fxfa/parser/cxfa_document.cpp
index 454a51c..9200ec2 100644
--- a/xfa/fxfa/parser/cxfa_document.cpp
+++ b/xfa/fxfa/parser/cxfa_document.cpp
@@ -1585,27 +1585,27 @@
                                                   bool bOneInstance,
                                                   bool bDataMerge,
                                                   bool bUpLevel) {
+  ASSERT(pTemplateNode->IsContainerNode());
   switch (pTemplateNode->GetElementType()) {
-    case XFA_Element::SubformSet:
-    case XFA_Element::Subform:
     case XFA_Element::Area:
     case XFA_Element::PageArea:
+    case XFA_Element::Subform:
+    case XFA_Element::SubformSet:
       return CopyContainer_SubformSet(this, pTemplateNode, pFormNode,
                                       pDataScope, bOneInstance, bDataMerge);
+    case XFA_Element::ContentArea:
+    case XFA_Element::Draw:
     case XFA_Element::ExclGroup:
     case XFA_Element::Field:
-    case XFA_Element::Draw:
-    case XFA_Element::ContentArea:
       return CopyContainer_Field(this, pTemplateNode, pFormNode, pDataScope,
                                  bDataMerge, bUpLevel);
     case XFA_Element::PageSet:
     case XFA_Element::Variables:
-      break;
+      return nullptr;
     default:
       NOTREACHED();
-      break;
+      return nullptr;
   }
-  return nullptr;
 }
 
 void CXFA_Document::DataMerge_UpdateBindingRelations(