On the Chromium side there was also a large refactor of SplitString.
This patch includes a replication of that refactor in string_utils and
adds the Chromium unittests to string_utils_unittests.cpp

BUG=angleproject:1234

Change-Id: I4f71064fbf325c204e98a7b36ead118913d90f2c
Reviewed-on: https://chromium-review.googlesource.com/314101Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>