Add extension string to registry
Add extension flag to EGL and Vulkan Displays

Bug: angleproject:3960
Test: dEQP-EGL.functional.surfaceless_context.*
      angle_end2end_tests --gtest_filter=EGLSurfacelessContextTest*
Change-Id: Id17916f1f636454af395b7bc92892dcf8fc93e54
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1885075
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>