Get rid of deprecated methods accessing mutator_thread_ instead of current thread

runtime/vm/object_graph.cc

Index: runtime/vm/object_graph.cc
diff --git a/runtime/vm/object_graph.cc b/runtime/vm/object_graph.cc
index 3a56dc9b5dcfbae14552cf5c4c9087fe2cd451a6..2bc0d664374374c5a5e9fcef582bf2c2c1e16bbf 100644
--- a/runtime/vm/object_graph.cc
+++ b/runtime/vm/object_graph.cc
@@ -156,12 +156,12 @@ class Unmarker : public ObjectVisitor {
 };
 
 
-ObjectGraph::ObjectGraph(Isolate* isolate)
-    : StackResource(isolate) {
+ObjectGraph::ObjectGraph(Thread* thread)
+    : StackResource(thread) {
   // The VM isolate has all its objects pre-marked, so iterating over it
   // would be a no-op.
-  ASSERT(isolate != Dart::vm_isolate());
-  isolate->heap()->WriteProtectCode(false);
+  ASSERT(thread->isolate() != Dart::vm_isolate());
+  thread->isolate()->heap()->WriteProtectCode(false);
 }