Enforce IWYU on ImageGenerator

Client CLs:
 - https://crrev.com/c/4303437
 - https://github.com/flutter/engine/pull/40026

Change-Id: Idcc4419e783130697ffcb6e9dff3b0f4ea2a08d0
Bug: skia:13983, skia:13052
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/651420
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
6 files changed