Return ImmutableString from ArrayString() This makes the compiler a few kilobytes smaller, and prepares getting rid of TString altogether. BUG=angleproject:2267 TEST=angle_unittests Change-Id: I93a003fe27b99bef72f872fa1066e2e108f934c5 Reviewed-on: https://chromium-review.googlesource.com/1107713 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>