Replace std::map with switch in load functions. BUG=angleproject:1455 Change-Id: Ica74ea5503efc0315bc4d98aa322da523a30b24c Reviewed-on: https://chromium-review.googlesource.com/367696 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>