Add stubs for CL platform layer back ends Bug: angleproject:5904 Change-Id: If23c7f76013a17c3c67c13194566438035beb3d1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2853582 Commit-Queue: John Plate <jplate@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com>