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

src/isolate.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdlib.h>
 
 #include <fstream>  // NOLINT(readability/streams)
 #include <sstream>
 
 #include "src/v8.h"
@@ skipping 2093 matching lines @@
     set_event_logger(Logger::DefaultEventLoggerSentinel);
   }
 
   // Set default value if not yet set.
   // TODO(yangguo): move this to ResourceConstraints::ConfigureDefaults
   // once ResourceConstraints becomes an argument to the Isolate constructor.
   if (max_available_threads_ < 1) {
     // Choose the default between 1 and 4.
     max_available_threads_ =
         Max(Min(base::SysInfo::NumberOfProcessors(), 4), 1);
+    if (FLAG_single_threaded) max_available_threads_ = 1;
   }
 
   if (FLAG_trace_hydrogen || FLAG_trace_hydrogen_stubs) {
     PrintF("Concurrent recompilation has been disabled for tracing.\n");
   } else if (OptimizingCompilerThread::Enabled(max_available_threads_)) {
     optimizing_compiler_thread_ = new OptimizingCompilerThread(this);
     optimizing_compiler_thread_->Start();
   }
 
   // Initialize runtime profiler before deserialization, because collections may
@@ skipping 516 matching lines @@
   if (prev_ && prev_->Intercept(flag)) return true;
   // Then check whether this scope intercepts.
   if ((flag & intercept_mask_)) {
     intercepted_flags_ |= flag;
     return true;
   }
   return false;
 }
 
 } }  // namespace v8::internal