Refactor 'dart:profiler' UserTag API

runtime/vm/tags.h

 // Copyright (c) 2013, 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.
 
 #ifndef VM_TAGS_H_
 #define VM_TAGS_H_
 
 #include "vm/allocation.h"
 
 namespace dart {
@@ skipping 65 matching lines @@
 
   void PrintToJSONObject(JSONObject* obj);
 
  private:
   int64_t counters_[VMTag::kNumVMTags];
 };
 
 
 class UserTags : public AllStatic {
  public:
-  static const uword kNoUserTag = 0;
   // UserTag id space: [kUserTagIdOffset, kUserTagIdOffset + kMaxUserTags).
   static const intptr_t kMaxUserTags = 64;
   static const uword kUserTagIdOffset = 0x4096;
+  static const uword kDefaultUserTag = kUserTagIdOffset;
   static const char* TagName(uword tag_id);
   static bool IsUserTag(uword tag_id) {
     return (tag_id >= kUserTagIdOffset) &&
            (tag_id < kUserTagIdOffset + kMaxUserTags);
   }
 };
 
 
 }  // namespace dart
 
 #endif  // VM_TAGS_H_