This will allow us to call the TexureStorage::setData easily.

BUG=angle:729

Change-Id: I16ada583894c8c427c78de5050f7f462c96d1fd1
Reviewed-on: https://chromium-review.googlesource.com/221492Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>