Use accessors to refer to the ModifiersPool.

This is a first step towards moving the ModifiersPool out of the
IRGenerator and into the Context.

Change-Id: I8000923f32a9e3cc1878a09464680884b950e513
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/404216
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
12 files changed