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>