[Parser] Don't mark receiver as MaybeAssigned since it can't be assigned.

Otherwise there is a mismatch between eager parsing (where the reciever
is marked as MaybeAssigned) and lazy parsing (where the receiver is
deserialized and not marked MaybeAssigned) for arrow functions that
have an inner scope that calls eval.

BUG=chromium:989914

Change-Id: I8b8b78140858985a75a971b0e0a95bd61463457b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1752851Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63206}

src/ast/scopes.cc

@@ -2070,7 +2070,7 @@ bool Scope::MustAllocate(Variable* var) {
   if (!var->raw_name()->IsEmpty() &&
       (inner_scope_calls_eval_ || is_catch_scope() || is_script_scope())) {
     var->set_is_used();
-    if (inner_scope_calls_eval_) var->SetMaybeAssigned();
+    if (inner_scope_calls_eval_ && !var->is_this()) var->SetMaybeAssigned();
   }
   DCHECK(!var->has_forced_context_allocation() || var->is_used());
   // Global variables do not need to be allocated.

test/mjsunit/regress/regress-989914.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --no-lazy --stress-lazy-source-positions
+
+function foo() {
+    return () => {
+      this.test_;
+      eval();
+  }
+}