Make Variable::is_this always return the correct value

src/full-codegen/mips64/full-codegen-mips64.cc

Index: src/full-codegen/mips64/full-codegen-mips64.cc
diff --git a/src/full-codegen/mips64/full-codegen-mips64.cc b/src/full-codegen/mips64/full-codegen-mips64.cc
index 103ea02ed5090acb1387d0530747b226918d2ba0..16115446c92be1338df76a48fef7f879fc149c4d 100644
--- a/src/full-codegen/mips64/full-codegen-mips64.cc
+++ b/src/full-codegen/mips64/full-codegen-mips64.cc
@@ -3069,7 +3069,7 @@ 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()) {
           __ LoadGlobalObject(a2);