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

src/flag-definitions.h

 // 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.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 731 matching lines @@
 
 // mksnapshot.cc
 DEFINE_STRING(startup_blob, NULL,
               "Write V8 startup blob file. (mksnapshot only)")
 
 // code-stubs-hydrogen.cc
 DEFINE_BOOL(profile_hydrogen_code_stub_compilation, false,
             "Print the time it takes to lazily compile hydrogen code stubs.")
 
 DEFINE_BOOL(predictable, false, "enable predictable mode")
-DEFINE_NEG_IMPLICATION(predictable, concurrent_recompilation)
-DEFINE_NEG_IMPLICATION(predictable, concurrent_osr)
-DEFINE_NEG_IMPLICATION(predictable, concurrent_sweeping)
+DEFINE_IMPLICATION(predictable, single_threaded)
+
+DEFINE_BOOL(single_threaded, false, "use a single thread to run")
+DEFINE_NEG_IMPLICATION(single_threaded, concurrent_recompilation)
+DEFINE_NEG_IMPLICATION(single_threaded, concurrent_osr)
+DEFINE_NEG_IMPLICATION(single_threaded, concurrent_sweeping)
 
 
 //
 // Dev shell flags
 //
 
 DEFINE_BOOL(help, false, "Print usage message, including flags, on console")
 DEFINE_BOOL(dump_counters, false, "Dump counters on exit")
 
 DEFINE_BOOL(debugger, false, "Enable JavaScript debugger")
@@ skipping 260 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA