Remove unnecessary isolate argument from some visitors

runtime/vm/pages.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/pages.h"
 
 #include "platform/assert.h"
 #include "vm/compiler_stats.h"
 #include "vm/gc_marker.h"
 #include "vm/gc_sweeper.h"
@@ skipping 680 matching lines @@
     space.AddProperty("avgCollectionPeriodMillis",
                       avg_time_between_collections);
   } else {
     space.AddProperty("avgCollectionPeriodMillis", 0.0);
   }
 }
 
 
 class HeapMapAsJSONVisitor : public ObjectVisitor {
  public:
-  explicit HeapMapAsJSONVisitor(JSONArray* array)
-      : ObjectVisitor(NULL), array_(array) {}
+  explicit HeapMapAsJSONVisitor(JSONArray* array) : array_(array) { }
   virtual void VisitObject(RawObject* obj) {
     array_->AddValue(obj->Size() / kObjectAlignment);
     array_->AddValue(obj->GetClassId());
   }
  private:
   JSONArray* array_;
 };
 
 
 void PageSpace::PrintHeapMapToJSONStream(
@@ skipping 517 matching lines @@
     return 0;
   } else {
     ASSERT(total_time >= gc_time);
     int result = static_cast<int>((static_cast<double>(gc_time) /
                                    static_cast<double>(total_time)) * 100);
     return result;
   }
 }
 
 }  // namespace dart