This gets rid of a few extra ImmutableString to std::string
conversions which makes the code clearer. There doesn't seem to be any
significant effect on performance.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I36ebbc8a4130ab8f9d6194feca83ec5dcbc95f78
Reviewed-on: https://chromium-review.googlesource.com/916141Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>