Revert "Remove kOverrideUseSoftwareGLForTests."

This reverts commit 5a1e5c5f.

Reason for revert: Caused tree closure due to build break: https://ci.chromium.org/ui/p/chromium/builders/ci/Linux%20ChromiumOS%20MSan%20Builder/49411/overview

Original change's description:
> Remove kOverrideUseSoftwareGLForTests.
>
> Instead, we should use --use-gl=angle --use-angle=swiftshader for
> tests that want to use SwiftShader instead of real GPU, for example, tests run on bots without real GPUs, but still want to exercise gpu accelerated paths instead of software fallback paths.
>
> Bug:1426236
> TEST=bots
>
> Change-Id: If6d7e88fb3bf626f56f8f8d75e9d5ad6b13f1768
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4355711


> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Commit-Queue: Zhenyao Mo <zmo@chromium.org>
> Reviewed-by: Ken Rockot <rockot@google.com>
> Cr-Commit-Position: refs/heads/main@{#1120020}

Bug: 1426236
Change-Id: I66d6b124a6de397acbae21b3e2885bfdbcf9a772
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4357756
Auto-Submit: Alison Gale <agale@chromium.org>
Owners-Override: Alison Gale <agale@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1120048}