Worker: Add UMA to record an exit code of WorkerThread

third_party/WebKit/Source/core/workers/WorkerThread.h

Index: third_party/WebKit/Source/core/workers/WorkerThread.h
diff --git a/third_party/WebKit/Source/core/workers/WorkerThread.h b/third_party/WebKit/Source/core/workers/WorkerThread.h
index 92c7dbe30e5e2029efd02695376b526fe3649d85..8800f462586a0b7f492c58f3a21fbaac36b9da2d 100644
--- a/third_party/WebKit/Source/core/workers/WorkerThread.h
+++ b/third_party/WebKit/Source/core/workers/WorkerThread.h
@@ -62,12 +62,13 @@ enum WorkerThreadStartMode {
 // an actual task is executed on the worker thread.
 class CORE_EXPORT WorkerThread {
 public:
-    // Represents how this thread is terminated.
+    // Represents how this thread is terminated. Used for UMA. Append only.
     enum class ExitCode {
         NotTerminated,
         GracefullyTerminated,
         SyncForciblyTerminated,
         AsyncForciblyTerminated,
+        LastEnum,
     };
 
     virtual ~WorkerThread();