Revert "Remove SkRuntimeShader::makeImage."

This reverts commit 65967ab2c9f6e9f3e667901bf86f4b791e0f782b.

Reason for revert: used in Android - http://screen/3fk6K3RUWZG8x5f

Original change's description:
> Remove SkRuntimeShader::makeImage.
>
> We don't have any known users, and it no longer exposes anything that
> a user can't just do directly.
>
> Change-Id: Id653a6be3f265a2847b1670f3e6c054cf2d094a2
> Bug: skia:12482
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/453142
> Auto-Submit: John Stiles <johnstiles@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: John Stiles <johnstiles@google.com>

Bug: skia:12482
Change-Id: I6cc2e79796f53c42e9f53b7d9927c853e7e2d71c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/453317
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Owners-Override: Kevin Lubick <kjlubick@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
3 files changed