This is a reland of 123fd970

Original change's description:
> Provide default implementation of rx::DisplayEGL
>
> Will allow to use EGL instead of GLX on X11.
>
> Meant to be used on modern EGL so it requires the extensions
> EGL_KHR_no_config_context and EGL_KHR_surfaceless_context.
> This keeps the default implementation simple (no pBuffer
> fallback)
>
> Also provide simple WorkerContextEGL.
>
> Bug: angleproject:4328
> Change-Id: I92a1ded9e09e65de8bb83710274e0a73ecdff0a4
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2061168
> Commit-Queue: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>

Bug: angleproject:4328
Change-Id: I02046613324e9283ba8f2ed02dd9c7ce703e0fde
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2063045Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>