In our EGL config we advertise a depth-stencil buffer but do not provide
it. This fixes a lot of end2end tests that were relying on the presence
of the depth buffer.

BUG=angleproject:891

Change-Id: I48e0eb27753ee64c08bf51b8ec13bfe20360332d
Reviewed-on: https://chromium-review.googlesource.com/298861Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>