GetSymbol previously always internalized the underlying string, even if the
preparser does not need it. The most common case where this isn't needed is
property name parsing. This seems to speed up preparsing quite a bit.

For future reference: Property names in object literals still are needed due to
various checks (e.g., get 'constructor', duplicate __proto__, ...); as well as
cover grammar parsing (property names can turn into variable references). If we
turn all strings that the preparser needs to identify back into contextual
keywords we may be able to avoid the former.

Change-Id: I549e4600053de3136ca08d4915cc04db36d66a89
Reviewed-on: https://chromium-review.googlesource.com/c/1488764Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59866}