[ Timeline ] Fix use-after-free error at shutdown when using endless timeline recorder

Timeline::Clear() results in the linked list of events being destroyed
when using the endless timeline recorder. This call was not protected by
RecorderLock::WaitForShutdown() and could result in an event being
allocated after the recorder's list of events had been destroyed.

Fixes #48899

TEST=CQ

Change-Id: I7e3de2bc99535a560321159ded70aef3e8686526
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/243140
Commit-Queue: Ben Konyi <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>

Author: bkonyi

runtime/vm/timeline.cc

@@ -222,8 +222,8 @@ void Timeline::Cleanup() {
   Timeline::stream_##name##_.set_enabled(false);
   TIMELINE_STREAM_LIST(TIMELINE_STREAM_DISABLE)
 #undef TIMELINE_STREAM_DISABLE
-  Timeline::Clear();
   RecorderLock::WaitForShutdown();
+  Timeline::Clear();
   delete recorder_;
   recorder_ = NULL;
   if (enabled_streams_ != NULL) {