The subclassing was not used by chromium and caused problems for users
that wanted to use a window owned by another thread as an EGL surface.

BUG=angleproject:961

Change-Id: I1d91323c758374de317179874320961385b10ae0
Reviewed-on: https://chromium-review.googlesource.com/266658Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: John Bauman <jbauman@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>