Move some functions to MathPriv.h

Change-Id: I22c57a746e4a997fc03f35aa06b23d9fa9ed4fa9
Reviewed-on: https://skia-review.googlesource.com/152183
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
7 files changed