SkPath::shrinkToFit docs

Add header and .bmh docs for shrinkToFit().

Also remove SkPath::debugging_private_getFreeSpace() and refactor tests
to use (friended) helper classes.

Docs-Preview: https://skia.org/?cl=153668
Change-Id: I3f93f9561b25025ce04a81d5e2f0271295b3daad
Reviewed-on: https://skia-review.googlesource.com/153668
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
5 files changed