Run Crankshaft on a separate thread.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 52a84edfdabf6e797b3d87cd1b2786eb6bfdd5a2..3c9d1f4e2bec447fd6c9aa831a69af68c26179e1 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -673,6 +673,14 @@ void V8::DisposeGlobal(i::Object** obj) {
 
 HandleScope::HandleScope() {
   i::Isolate* isolate = i::Isolate::Current();
+  if (i::FLAG_optimize_in_parallel &&
+      !isolate->thread_manager()->IsLockedByCurrentThread()) {
+    has_isolate_lock_ = true;
+    isolate->thread_manager()->Lock();
+  } else {
+    has_isolate_lock_ = false;
+  }
+
   API_ENTRY_CHECK(isolate, "HandleScope::HandleScope");
   v8::ImplementationUtilities::HandleScopeData* current =
       isolate->handle_scope_data();
@@ -688,6 +696,9 @@ HandleScope::~HandleScope() {
   if (!is_closed_) {
     Leave();
   }
+  if (has_isolate_lock_) {
+    isolate_->thread_manager()->Unlock();
+  }
 }