This reverts commit f6e73131.

Reason for revert: Crashes on Win10 FYI x64 Debug (NVIDIA) and Win7 FYI Debug (AMD) in the webgl CTS

Original change's description:
> Move sampleAlphaToCoverage out of blendState
> 
> This is the second step towards exposing OES_draw_buffers_indexed (that defines independent blend state for each draw buffer). This flag is global in all graphics APIs, however D3D11 technically puts it in the blend state.
> 
> D3D11: BlendStateKey was extended to keep existing D3D11 state caching semantics.
> 
> D3D9: a comment was added explaining why this feature was never implemented there.
> 
> Bug: angleproject:4394
> Change-Id: Ie6a294eeb6fcf4c868a1f1001c4f7efd61692ccd
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057063
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Commit-Queue: Geoff Lang <geofflang@chromium.org>

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

Change-Id: I650624b5dfb7f2777c316906b9145a411243f42f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: angleproject:4394
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2062605Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>