This reverts commit c746ac65.

Reason for revert: Possible performance regression
BUG=chromium:1085996

Original change's description:
> Remove redundant BlendStateArray tracking
>
> Keep legacy BlendState for now.
>
> Bug: angleproject:4394
> Change-Id: Icba2b2f3a071d0f838a5480ff94869d35b776d94
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2169093
> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
> Commit-Queue: Kenneth Russell <kbr@chromium.org>

TBR=geofflang@chromium.org,kbr@chromium.org,jonahr@google.com,jmadill@chromium.org,lexa.knyazev@gmail.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: angleproject:4394
Change-Id: Id05b382e951a7256805cffe696325b6b6d940e96
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2246719
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>