Rename GrDrawContext to GrRenderTargetContext

This is in preparation for GrTextureContext and GrSurfaceContext

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4030

Change-Id: Ie58c93052e68f3f1f5fe8d15d63760de274a6fbd
Reviewed-on: https://skia-review.googlesource.com/4030
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
109 files changed