Added autogeneration of method declarations for GLES2+ and extensions

Bug: angleproject:3526
Change-Id: I9c7263452146098512d9584deae8ea3f15f62c46
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1660949Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>