This will allow a more flexible introduction of other ICDs like
SwiftShader. Refactoring change only.

Bug: angleproject:3876
Change-Id: I6883225645e0f961f699366368bebccd9812aaec
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1775463
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>