[parser] Pass PreParserIdentifier / Expression by const&.

Now that they contain more than just one member variable, this might make
sense (and at least doesn't do any harm).

BUG=v8:6092

Change-Id: Ibd150bedf542580f69946f62f8aca47bce945499
Reviewed-on: https://chromium-review.googlesource.com/623748
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47547}

src/parsing/preparser.cc

@@ -396,7 +396,7 @@ PreParserStatement PreParser::BuildParameterInitializationBlock(
 }
 
 PreParserExpression PreParser::ExpressionFromIdentifier(
-    PreParserIdentifier name, int start_position, InferName infer) {
+    const PreParserIdentifier& name, int start_position, InferName infer) {
   VariableProxy* proxy = nullptr;
   if (track_unresolved_variables_) {
     DCHECK_NOT_NULL(name.string_);