This allows us to return multiple EGL configs, in particular we can have
configs with no depth-stencil buffer. Also includes a description of the
investigation I did for this patch.

BUG=angleproject:892

Change-Id: I921e254e91e818396dd4c9bd0ee2a43b00a043b9
Reviewed-on: https://chromium-review.googlesource.com/271426Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>