We can reduce the code in these classes quite a bit by calling
the common code in egl::Display.

BUG=angleproject:994

Change-Id: Idf0ffa054fc6a70cf8c65a0b98fd963911715591
Reviewed-on: https://chromium-review.googlesource.com/269258Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>