Migrate ModifiersPool into the context.

The IRGenerator no longer owns the pool of modifiers; they are now
available to any function that has a Context.

Change-Id: I4f2f37ae5e06966bed00107b0483b42b07a454d8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/404237
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
13 files changed