Update CGL backend to use new EGL types.

BUG=angleproject:1707

Change-Id: I165e81a758cad0a8036aef68c44a57bcc61e982d
Reviewed-on: https://chromium-review.googlesource.com/428292Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>