Define EGL_PLATFORM_ANGLE_DEVICE_TYPE_EGL_ANGLE that should be
used to request a driver egl-backed implementation with ANGLE.

Bug: angleproject:4328
Change-Id: I6871d3a27e2bfc02af9815dcf86ae1cb6524f0cc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2030038
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>