Minor simplification and cleanup of graph builder.

src/compiler/pipeline.cc

Index: src/compiler/pipeline.cc
diff --git a/src/compiler/pipeline.cc b/src/compiler/pipeline.cc
index 1752882779c78fe7b2f3baf11e6172a140e9083f..ca47b70bef7bc889ec94265b3f68c96ce931f467 100644
--- a/src/compiler/pipeline.cc
+++ b/src/compiler/pipeline.cc
@@ -85,16 +85,25 @@ class AstGraphBuilderWithPositions : public AstGraphBuilder {
  public:
   explicit AstGraphBuilderWithPositions(CompilationInfo* info, JSGraph* jsgraph,
                                         SourcePositionTable* source_positions)
-      : AstGraphBuilder(info, jsgraph, source_positions) {}
+      : AstGraphBuilder(info, jsgraph), source_positions_(source_positions) {}
+
+  bool CreateGraph() V8_OVERRIDE {
+    SourcePositionTable::Scope pos(source_positions_,
+                                   SourcePosition::Unknown());
+    return AstGraphBuilder::CreateGraph();
+  }
 
 #define DEF_VISIT(type)                                               \
   virtual void Visit##type(type* node) V8_OVERRIDE {                  \
-    SourcePositionTable::Scope pos(source_positions(),                \
+    SourcePositionTable::Scope pos(source_positions_,                 \
                                    SourcePosition(node->position())); \
     AstGraphBuilder::Visit##type(node);                               \
   }
   AST_NODE_LIST(DEF_VISIT)
 #undef DEF_VISIT
+
+ private:
+  SourcePositionTable* source_positions_;
 };