Reland "Merge WEBGL_multi_draw_instanced into WEBGL_multi_draw"

This is a reland of 32cfad6080b077e2e9c8238ecd2c44a0933917d5
The original CL was reverted because it uncovered a state leak
elsewhere in ANGLE.

Original change's description:
> Merge WEBGL_multi_draw_instanced into WEBGL_multi_draw
>
> The WebGL working group decided there is enough WebGL 1 support for
> instancing that it would be simpler to only expose the instanced
> version of the extension.
>
> This also rolls WebGL 2701c13..69e333c which includes the spec and
> conformance test changes.
>
> Bug: chromium:890539
> Change-Id: Ib4dccf56f488cedf8161268ac3467d27c9b371c9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1881712
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Kenneth Russell <kbr@chromium.org>
> Commit-Queue: Austin Eng <enga@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#710310}

Bug: chromium:890539, chromium:1022080
Change-Id: I25fce02e99f0298b2f585f6ecdf47c0a0d0657d7
TBR: dcheng@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890541
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714084}
20 files changed