This is a small refactor to allow State::initialize to be called without
needing the EGL context creation attributes.

BUG=angleproject:2464

Change-Id: Ifa167cc83f652435ecc00a0a73e4c6c4a3295430
Reviewed-on: https://chromium-review.googlesource.com/1083312Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>