Change interface block's fVariable from ref to pointer

The next pre-include refactor will require cloning InterfaceBlocks, and
re-targeting the variable.

Change-Id: Iccfc1f39789fcd572199682386cd612500334061
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/323890
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
6 files changed