Make AstGraphBuilder::function_context a SetOncePointer again.

src/compiler/ast-graph-builder.h

Index: src/compiler/ast-graph-builder.h
diff --git a/src/compiler/ast-graph-builder.h b/src/compiler/ast-graph-builder.h
index 515283fc8a48fabde62cce0b7959e3e1552b98fa..7ef58a2713a31f92995ac0aa3999134aba6ba734 100644
--- a/src/compiler/ast-graph-builder.h
+++ b/src/compiler/ast-graph-builder.h
@@ -88,7 +88,7 @@ class AstGraphBuilder : public AstVisitor {
 
   // Nodes representing values in the activation record.
   SetOncePointer<Node> function_closure_;
-  Node* function_context_;
+  SetOncePointer<Node> function_context_;
 
   // Temporary storage for building node input lists.
   int input_buffer_size_;
@@ -174,6 +174,7 @@ class AstGraphBuilder : public AstVisitor {
   Node* NewPhi(int count, Node* input, Node* control);
   Node* NewEffectPhi(int count, Node* input, Node* control);
 
+  // Creates new context nodes.
   Node* NewOuterContextParam();
   Node* NewCurrentContextOsrValue();