Add helper function to pool allocate a char array This makes it easier to allocate char arrays in the memory pool. TEST=angle_unittests BUG=angleproject:2267 Change-Id: I1673330f58968ea9d38c671b70a7a489276af863 Reviewed-on: https://chromium-review.googlesource.com/883805 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>