Make GrDrawContext be GrRenderTargetProxy-backed

Make GrDrawContext be GrRenderTargetProxy-backed

This is split out of https://codereview.chromium.org/2215323003/ (Start using RenderTargetProxy (omnibus))

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3841

CQ_INCLUDE_TRYBOTS=master.client.skia.android:Test-Android-Clang-Nexus5-GPU-Adreno330-arm-Debug-GN_Android-Trybot

Change-Id: I1a47f19ed1ac0c249e6ccac8db74095d7f456db4
Reviewed-on: https://skia-review.googlesource.com/3841
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>