Infer NodeType in attachDiscardableAdapter.

This makes the call sites a bit simpler, doesn't materially change
anything in an optimized build, allows NRVO, and generally fixes a
number of warnings in gcc 9 about pessimizing-move.

Change-Id: I0ea5f57db163425da728630bfa6c1add7c416bd7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278178
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Ben Wagner <bungeman@google.com>
8 files changed