Refactor GLES2 Blend entry points. BUG=angleproject:747 Change-Id: I775265a7494aedd19ca4f18019e3b3181bfb12e9 Reviewed-on: https://chromium-review.googlesource.com/410135 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>