Don't define SkSurface::MakeRenderTarget when there is no GPU backend

Follow up on Android TODO in SkSurface

Client CLs:
 - http://ag/22965803
 - http://ag/22989547
 - https://crrev.com/c/4503058

Change-Id: I8ba72c2c60c9c1ca2fa2a55e720e9eabc99451ad
Bug: skia:13983
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/687779
Reviewed-by: Leon Scroggins <scroggo@google.com>
Owners-Override: Kevin Lubick <kjlubick@google.com>
2 files changed