Support OES_sample_shading extension if the underlying
Vulkan ICD supports sampleRateShading.

Bug: angleproject:3587
Tests: dEQP-GLES31.functional.sample_shading.*
Change-Id: I1b324c1ad3ea3b2157d3cbe0abcdf7085aa4231b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2444213
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>