Update EGLWindow to select configs that exactly match the requested
config.  Too many tests make assumptions about the default framebuffer's
format.

BUG=angleproject:1662

Change-Id: I4cf8aedf51013ca0f3f40f6bbd40f24a4a90561f
Reviewed-on: https://chromium-review.googlesource.com/419681Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>