Avoid data race in debug mode on the parallel thread.

src/optimizing-compiler-thread.cc

Index: src/optimizing-compiler-thread.cc
diff --git a/src/optimizing-compiler-thread.cc b/src/optimizing-compiler-thread.cc
index b9ff7d83eca369c0b194ab2cd40ccdec5c9379be..dbf9ad7cdcdb57e2455fb89d58cf87b3d4227197 100644
--- a/src/optimizing-compiler-thread.cc
+++ b/src/optimizing-compiler-thread.cc
@@ -39,7 +39,9 @@ namespace internal {
 
 void OptimizingCompilerThread::Run() {
 #ifdef DEBUG
-  thread_id_ = ThreadId::Current().ToInteger();
+  { ScopedLock lock(thread_id_mutex_);
+    thread_id_ = ThreadId::Current().ToInteger();
+  }
 #endif
   Isolate::SetIsolateThreadLocals(isolate_, NULL);
   DisallowHeapAllocation no_allocation;
@@ -156,6 +158,7 @@ void OptimizingCompilerThread::QueueForOptimization(
 #ifdef DEBUG
 bool OptimizingCompilerThread::IsOptimizerThread() {
   if (!FLAG_parallel_recompilation) return false;
+  ScopedLock lock(thread_id_mutex_);
   return ThreadId::Current().ToInteger() == thread_id_;
 }
 #endif