We can just expose OES_texture_storage_multisample_2d_array instead.
The compiler was already changed to accept
OES_texture_storage_multisample_2d_array, and now the change is made
also at the API level.

Out-of-bounds access guarantees provided by ANGLE were the only big
difference between the ANGLE spec and the OES spec, so it's simpler
to just expose the native extension. Safe out-of-bounds accesses can
be guaranteed without having them in the extension spec.

This also adds missing texStorage3DMultisample entry point to the proc
table, which will enable running dEQP tests.

BUG=angleproject:2775
TEST=angle_end2end_tests

Change-Id: Idf376ee877a3374a33de177df023f0531ec8f01d
Reviewed-on: https://chromium-review.googlesource.com/1196722Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>