This refactors the auto-generation script to use a simpler XML
iteration. It will only query the Xpath once per script, instead
of once per entry point. This speeds up execution significantly.

Also this change sorts the entry points alphabetically instead
of having them appear in the order they appear in the XML. This
gives a more consistent ordering.

Bug: angleproject:1309
Change-Id: Ifa1110af786b91ad0e6ff1cd3707e17666d398a5
Reviewed-on: https://chromium-review.googlesource.com/846419Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>