Addition of just the entry points for OES_sample_shading extension.

Bug: angleproject:3587
Change-Id: I7ef5ceb846d130c2d1e0b4ea7dcbe92452b5d7d3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2444212
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>