Make assertion scopes thread safe.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index c94af7d909734ff1554e1a311bfacb0f02117ced..cb3cd44b6709feda586d1d3a28515e4105acecab 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -837,8 +837,7 @@ void LCodeGen::PopulateDeoptimizationData(Handle<Code> code) {
 
   Handle<FixedArray> literals =
       factory()->NewFixedArray(deoptimization_literals_.length(), TENURED);
-  { ALLOW_HANDLE_DEREF(isolate(),
-                       "copying a ZoneList of handles into a FixedArray");
+  { AllowDeferredHandleDereference copy_handles;
     for (int i = 0; i < deoptimization_literals_.length(); i++) {
       literals->set(i, *deoptimization_literals_[i]);
     }
@@ -1588,7 +1587,7 @@ void LCodeGen::DoConstantD(LConstantD* instr) {
 
 void LCodeGen::DoConstantT(LConstantT* instr) {
   Handle<Object> value = instr->value();
-  ALLOW_HANDLE_DEREF(isolate(), "smi check");
+  AllowDeferredHandleDereference smi_check;
   if (value->IsSmi()) {
     __ Move(ToRegister(instr->result()), value);
   } else {
@@ -5336,7 +5335,7 @@ void LCodeGen::EmitPushTaggedOperand(LOperand* operand) {
   ASSERT(!operand->IsDoubleRegister());
   if (operand->IsConstantOperand()) {
     Handle<Object> object = ToHandle(LConstantOperand::cast(operand));
-    ALLOW_HANDLE_DEREF(isolate(), "smi check");
+    AllowDeferredHandleDereference smi_check;
     if (object->IsSmi()) {
       __ Push(Handle<Smi>::cast(object));
     } else {