By making mData public we can use aggregate initialization. The syntax
is the same as for initializing a std::array. It might be possible to
also use std::map initialization syntax using a list of pairs. That is
left for a follow-up.

Bug: angleproject:2568
Change-Id: I30a6f280172cb197208d14b1a53b9cc12b7834d8
Reviewed-on: https://chromium-review.googlesource.com/1127181
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>