This is a reland of 8b9889bf

The previous CL relied on a define which now only exists in
a specific gn configuration. This reland removes D3D11 as a
multithread-supported platform in the test configuration.

Original change's description:
> Fix multithreaded crash on draw commands on D3D11 backend.
>
> A crash can occur if thread A is executing eglDestroyContext while
> thread B issues a draw call, if the threads are interleaved in such a
> manner that a makeCurrent occurs without triggering a change to the
> global context and a dirtyAllState call. We handle that case by
> explicitly making current the proper contexts in the eglDestroyContext
> call.
>
> A test has been added that triggers a crash without this fix when
> running on the D3D11 backend. In addition, all of MultithreadingTest
> is enabled for the D3D11 backend.
>
> Test: Ran MultithreadingTest. Test exhibits a crash before this
> change, and does not after this change. Also ran:
>     dEQP-EGL.functional.sharing.gles2.multithread.*
>     dEQP-EGL.functional.multithread.*
>
> Bug: b/183756357
> Change-Id: Ic6f76a062868b2f3b4e60d29dc087ec180bfb7cd
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2798591
> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Doug Horn <doughorn@google.com>

Bug: b/183756357
Change-Id: I5be9a011ea99a69730eddc9e4da23bcf92ed3bf2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2815243Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Doug Horn <doughorn@google.com>