Addition of the entry points for EXT_texture_border_clamp extension.

Bug: angleproject:3586
Change-Id: I366c2748213d7fdde815543129320278937000e0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2830146
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>