This change is pure refactoring. It's intended to help with adding
support for arrays of arrays.

BUG=angleproject:2125
TEST=angle_unittests

Change-Id: I82881a98c3c476fd6666a551ce6be255ae0de4cf
Reviewed-on: https://chromium-review.googlesource.com/733127Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>