Fix crash of MultithreadingTest end2end tests.
Fix crash of MultithreadingTest end2end tests.

Extension platformANGLEContextVirtualization is currently only
supported in the OpenGL backend, it's confused to validate it
when the test is set up by WithNoVirtualContexts(). It'll generate
crash when it's not OpenGL backend.

Bug: angleproject:5636
Change-Id: Ib04fb4a3d29beaac5162e63af5ef1fd80c8a420c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2709821
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Shahbaz Youssefi <582eeec4f08a2ccd61e3ef52115e6e7c571582de@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle