Add a command line flag to disable streaming.

It's useful for debugging / analytics purposes, since we can force all
parsing to happen on a foreground thread.

BUG=

Review-Url: https://codereview.chromium.org/2386123006
Cr-Commit-Position: refs/heads/master@{#39977}

src/api.cc

@@ -2215,6 +2215,9 @@ Local<Function> ScriptCompiler::CompileFunctionInContext(
 
 ScriptCompiler::ScriptStreamingTask* ScriptCompiler::StartStreamingScript(
     Isolate* v8_isolate, StreamedSource* source, CompileOptions options) {
+  if (!i::FLAG_script_streaming) {
+    return nullptr;
+  }
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
   return new i::BackgroundParsingTask(source->impl(), options,
                                       i::FLAG_stack_size, isolate);

src/flag-definitions.h

@@ -591,6 +591,9 @@ DEFINE_MAYBE_BOOL(enable_armv8, "deprecated (use --arm_arch instead)")
 DEFINE_BOOL(enable_regexp_unaligned_accesses, true,
             "enable unaligned accesses for the regexp engine")
 
+// api.cc
+DEFINE_BOOL(script_streaming, true, "enable parsing on background")
+
 // bootstrapper.cc
 DEFINE_STRING(expose_natives_as, NULL, "expose natives in global object")
 DEFINE_STRING(expose_debug_as, NULL, "expose debug in global object")