Rename renderTargetContextPriv to priv, and some more leftover DCs

API change is for non-public API.

TBR=bsalomon@google.com

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4060

Change-Id: I5011d753c3c6d1145ff242eab6baff2ae0647ba3
Reviewed-on: https://skia-review.googlesource.com/4060
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
19 files changed