This reverts commit 28c5ad9b.

Reason for revert: suspect culprit for PrecisionSamplerShadersCompile/PrecisionSamplerShaderPixelTest.ShadersCompile/* failures on linux-tests-dbg: https://ci.chromium.org/p/chromium/builders/ci/Linux%20Tests%20%28dbg%29%281%29/93470

Original change's description:
> Switch cc_unittests/viz_unittests to SwANGLE
>
> This cl changes the software backend used for
> running viz_unittests and cc_unittests from
> SwiftShader GL to SwANGLE (ANGLE on SwiftShader
> Vulkan). Other tests suites may be affected.
>
> From this cl, performance expectations for test
> runs will likely change. For example, cc_unittests
> appears to run faster, while viz_unittests and
> views_unittests appear to run slower. Note that
> this is still using the validating command decoder,
> and once the OOP Raster tests can run on the
> passthrough command decoder, we'll be able to use
> it and gain some performance on all these test runs.
>
> Bug: chromium:1060139
> Change-Id: Ia243f05f3e9f54dc6f6dfaa3041021fc28818b2f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532042
> Commit-Queue: Alexis Hétu <sugoi@chromium.org>
> Reviewed-by: weiliangc <weiliangc@chromium.org>
> Reviewed-by: Kenneth Russell <kbr@chromium.org>
> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
> Cr-Commit-Position: refs/heads/master@{#834246}

TBR=sugoi@chromium.org,kbr@chromium.org,weiliangc@chromium.org,khushalsagar@chromium.org,jonahr@google.com,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com,sugoi@google.com

Change-Id: I815d610741c258a66dc360b2d302d2f663a3bf4c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1060139
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577957Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834373}