This improves encapsulation inside the Texture class, and removes
duplication of createTexture calls. This is a necessary step towards
adding a shared "Data" structure to the Texture classes, following a
similar pattern as for example the Framebuffer class.

This patch also shares the same MockFactory class among different
unit tests.

BUG=angleproject:596
TEST=angle_unittests

Change-Id: Ie8d3a9aa4ec35565d7ecbabb8c40e7b1ba068721
Reviewed-on: https://chromium-review.googlesource.com/340200Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>