Keep legacy BlendState for now.

Bug: angleproject:4394
Change-Id: Icba2b2f3a071d0f838a5480ff94869d35b776d94
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2169093Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Kenneth Russell <kbr@chromium.org>