Refactor 'dart:profiler' UserTag API

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index c090162049cddf54853a78c16fa3155928c0bb47..d185883e0c78c653bec8544729f655de0d5ed127 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -343,7 +343,7 @@ Isolate::Isolate()
       REUSABLE_HANDLE_LIST(REUSABLE_HANDLE_SCOPE_INIT)
       reusable_handles_() {
   set_vm_tag(VMTag::kIdleTagId);
-  set_user_tag(UserTags::kNoUserTag);
+  set_user_tag(UserTags::kDefaultUserTag);
 }
 #undef REUSABLE_HANDLE_SCOPE_INIT
 #undef REUSABLE_HANDLE_INITIALIZERS
@@ -1013,12 +1013,6 @@ void Isolate::set_current_tag(const UserTag& tag) {
 }
 
 
-void Isolate::clear_current_tag() {
-  set_user_tag(UserTags::kNoUserTag);
-  current_tag_ = UserTag::null();
-}
-
-
 void Isolate::VisitIsolates(IsolateVisitor* visitor) {
   if (visitor == NULL) {
     return;