[preparser] Drop tracking this as possible declaration

|this| cannot be an ambiguous declaration. Actual declarations are inserted
upon function scope creation, so we can simply parse as reference and it will
resolve correctly.

Change-Id: I3aaa1a2666b4caffbf8524caec7068125e10240b
Reviewed-on: https://chromium-review.googlesource.com/1251162Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56283}

src/parsing/preparser.h

@@ -171,10 +171,9 @@ class PreParserExpression {
                                IsUseAsmField::encode(true));
   }
 
-  static PreParserExpression This(VariableZoneThreadedListType* variables) {
+  static PreParserExpression This() {
     return PreParserExpression(TypeField::encode(kExpression) |
-                                   ExpressionTypeField::encode(kThisExpression),
-                               variables);
+                               ExpressionTypeField::encode(kThisExpression));
   }
 
   static PreParserExpression ThisPropertyWithPrivateFieldKey() {
@@ -1612,17 +1611,12 @@ class PreParser : public ParserBase<PreParser> {
   }
 
   V8_INLINE PreParserExpression ThisExpression(int pos = kNoSourcePosition) {
-    PreParserExpression::VariableZoneThreadedListType* variables = nullptr;
     if (track_unresolved_variables_) {
-      VariableProxy* proxy = scope()->NewUnresolved(
-          factory()->ast_node_factory(), ast_value_factory()->this_string(),
-          pos, THIS_VARIABLE);
-
-      variables =
-          new (zone()) PreParserExpression::VariableZoneThreadedListType();
-      variables->Add(proxy);
+      scope()->NewUnresolved(factory()->ast_node_factory(),
+                             ast_value_factory()->this_string(), pos,
+                             THIS_VARIABLE);
     }
-    return PreParserExpression::This(variables);
+    return PreParserExpression::This();
   }
 
   V8_INLINE PreParserExpression NewSuperPropertyReference(int pos) {