Migrate most uses of Isolate::current_zone to Thread::zone.

runtime/vm/timeline.cc

Index: runtime/vm/timeline.cc
diff --git a/runtime/vm/timeline.cc b/runtime/vm/timeline.cc
index 2fc14408b2754e9b4dbbc306b08e3e8b45d8b7ca..3d5e1798d86ae2262e871bf81438c9de09781b1c 100644
--- a/runtime/vm/timeline.cc
+++ b/runtime/vm/timeline.cc
@@ -371,7 +371,7 @@ void TimelineEventRecorder::WriteTo(const char* directory) {
   intptr_t pid = OS::ProcessId();
   intptr_t len = OS::SNPrint(NULL, 0, format,
                              directory, pid, isolate->main_port());
-  char* filename = isolate->current_zone()->Alloc<char>(len + 1);
+  char* filename = Thread::Current()->zone()->Alloc<char>(len + 1);
   OS::SNPrint(filename, len + 1, format,
               directory, pid, isolate->main_port());
   void* file = (*file_open)(filename, true);