[turbofan] Allocate SourcePositionTable inside a Zone.

src/compiler/source-position.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_SOURCE_POSITION_H_
 #define V8_COMPILER_SOURCE_POSITION_H_
 
 #include "src/assembler.h"
 #include "src/compiler/node-aux-data.h"
 
@@ skipping 48 matching lines @@
     void Init(SourcePosition position) {
       if (position.IsKnown()) source_positions_->current_position_ = position;
     }
 
     SourcePositionTable* const source_positions_;
     SourcePosition const prev_position_;
     DISALLOW_COPY_AND_ASSIGN(Scope);
   };
 
   explicit SourcePositionTable(Graph* graph);
-  ~SourcePositionTable() {
-    if (decorator_) RemoveDecorator();
-  }
 
   void AddDecorator();
   void RemoveDecorator();
 
   SourcePosition GetSourcePosition(Node* node) const;
 
   void Print(std::ostream& os) const;
 
  private:
   class Decorator;
 
   Graph* const graph_;
   Decorator* decorator_;
   SourcePosition current_position_;
   NodeAuxData<SourcePosition> table_;
 
   DISALLOW_COPY_AND_ASSIGN(SourcePositionTable);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_SOURCE_POSITION_H_