Implement Sk2f::floor

Bug: skia:
Change-Id: Id40e7165a338d321df71a1852b48eb2570ecd75b
Reviewed-on: https://skia-review.googlesource.com/133460
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed