Control access to adding ref to a GrGpuResource when it's ref count is zero.

We want GrResourceCache to be able to track which resources are held by
refs (as opposed to pending IOs) so that it can track the affect of
flushing on resource purgeability. Therefore, all cases that can add the
first ref to a GrGpuResource must funnel through GrResourceCache. This
lays the groundwork by restricting initial refs.

No-Presubmit: true
No-Tree-Checks: true
No-Try: true

Bug: skia:8927
Change-Id: I1213c3db258d2412df6666e3222419211ceaa192
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205482
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed