Disabling SwiftShader in the GN build led to a compile error
when trying to use ANGLE_VK_SWIFTSHADER_ICD_JSON.

Bug: angleproject:4223
Change-Id: I076a88e6bc58adf1e34bbb7286b1f46028cd021a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1969059
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>