Change comments to refer to "render target context" rather than "draw context"

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4180

Change-Id: I553043ed576d0654faa68ed2e9e9978e4fc6476c
Reviewed-on: https://skia-review.googlesource.com/4180
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed