+ adds query for GL_MAX_TEXTURE_UNITS

BUG=angleproject:2306

Change-Id: Ie89fa6a067551170856bf0f7e6d7b4452b3da132
Reviewed-on: https://chromium-review.googlesource.com/984894
Commit-Queue: Lingfeng Yang <lfy@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>