[skottie] Clang-tidy fixes

Mostly const-relax to allow moving.  Also add an r-val overload for
Node attribute setters.

Change-Id: I15e5eeee95b7c2a460844fec5a5d312d59b612ca
Reviewed-on: https://skia-review.googlesource.com/c/163168
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed