Add extension string and flags
Add EGLReusableSync files to libANGLE and gni

Test: angle_deqp_egl_tests
--deqp-case=dEQP-EGL.functional.reusable_sync.*
--deqp-case=dEQP-EGL.functional.fence_sync.*

Bug: angleproject:5168
Change-Id: I967a10cf387047c9ee1963acfc854a8288325a8e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2477293
Commit-Queue: Jeff Vigil <j.vigil@samsung.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>