add constexpr to SkRect, SkIRect Make functions

Also, doing so exposed a couple of unused
variables in tests.

R: bsalomon@google.com
Bug: skia: 6898
Change-Id: I7b065e26a838fe55a1d772bcefaef5325e1baa61
Reviewed-on: https://skia-review.googlesource.com/55680
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
3 files changed