There's an OES extension for multisample texture arrays,
OES_texture_storage_multisample_2d_array. Change references from
ANGLE_texture_multisample_array to the native extension in the shader
compiler.

ANGLE still needs to have robust behavior for out-of-range texel
fetches that's not found in the original extension, but this does not
need to be spelled out in the extension spec.

BUG=angleproject:2775
TEST=angle_unittests

Change-Id: Ie80ae767cc92ccaf7389af28789f45547f86978f
Reviewed-on: https://chromium-review.googlesource.com/1193266
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>