RenderTargetContext was renamed several months ago; this CL just fixes
up a few related names which hadn't yet updated yet.
Change-Id: I59a6ea92fc5d309a9b45a83e3aca6e49cb8d662e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/415745
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>