Enable context virtualization only for GL

Only the GL back-end requires context virtualization. So only
require it for that case and not for Vulkan back-end.

Test: dEQP with ANGLE and ANGLE build with args.gn with
angle_enable_gl=false
Bug: 150892231

(cherry picked from commit 559b4339)

Change-Id: I443841c2543d0c6446d0f11122bf18dc9acbf428