Remove more uses of Isolate::current_zone.

runtime/vm/handles.cc

Index: runtime/vm/handles.cc
diff --git a/runtime/vm/handles.cc b/runtime/vm/handles.cc
index f636fdd29171a1d6d6747da37717c655090f22c7..c0535423cc27528ff9a15a1fbc351812bd6aec4e 100644
--- a/runtime/vm/handles.cc
+++ b/runtime/vm/handles.cc
@@ -76,17 +76,17 @@ bool VMHandles::IsZoneHandle(uword handle) {
 
 
 int VMHandles::ScopedHandleCount() {
-  Isolate* isolate = Isolate::Current();
-  ASSERT(isolate->current_zone() != NULL);
-  VMHandles* handles = isolate->current_zone()->handles();
+  Thread* thread = Thread::Current();
+  ASSERT(thread->zone() != NULL);
+  VMHandles* handles = thread->zone()->handles();
   return handles->CountScopedHandles();
 }
 
 
 int VMHandles::ZoneHandleCount() {
-  Isolate* isolate = Isolate::Current();
-  ASSERT(isolate->current_zone() != NULL);
-  VMHandles* handles = isolate->current_zone()->handles();
+  Thread* thread = Thread::Current();
+  ASSERT(thread->zone() != NULL);
+  VMHandles* handles = thread->zone()->handles();
   return handles->CountZoneHandles();
 }