Make Variable::is_this always return the correct value

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index a9c20dc84c98c03ad0b7156f274414b67228b9c5..f9de2081c4ae651b6275bf018cf4a01e1eef0b2d 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -2948,7 +2948,7 @@ void FullCodeGenerator::VisitUnaryOperation(UnaryOperation* expr) {
         Variable* var = proxy->var();
         // Delete of an unqualified identifier is disallowed in strict mode but
         // "delete this" is allowed.
-        bool is_this = var->HasThisName(isolate());
+        bool is_this = var->is_this();
         DCHECK(is_sloppy(language_mode()) || is_this);
         if (var->IsUnallocatedOrGlobalSlot()) {
           __ movp(rax, NativeContextOperand());