Add --single-threaded flag that disables background threads.

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 5999df9d6d724916c4c3d5d99f177e0e29c70a5c..c83963a32fa9bd5a9c5f0b1ce01e2bc13a35b1ae 100644
--- a/src/optimizing-compiler-thread.cc
+++ b/src/optimizing-compiler-thread.cc
@@ -372,6 +372,7 @@ void OptimizingCompilerThread::QueueForOptimization(OptimizedCompileJob* job) {
   if (FLAG_block_concurrent_recompilation) {
     blocked_jobs_++;
   } else if (job_based_recompilation_) {
+    DCHECK(!FLAG_single_threaded);
     base::LockGuard<base::RecursiveMutex> lock(&task_count_mutex_);
     ++task_count_;
     V8::GetCurrentPlatform()->CallOnBackgroundThread(
@@ -390,6 +391,7 @@ void OptimizingCompilerThread::Unblock() {
   }
   while (blocked_jobs_ > 0) {
     if (job_based_recompilation_) {
+      DCHECK(!FLAG_single_threaded);
       V8::GetCurrentPlatform()->CallOnBackgroundThread(
           new CompileTask(isolate_), v8::Platform::kShortRunningTask);
     } else {