make kPathRefGenIDBitCnt private

kPathRefGenIDBitCnt is used only by SkPath.cpp and SkPathRef.cpp.
Curiously, there's no existing private declarations shared by
SkPath and SkPathRef, so I added kPathRefGenIDBitCnt to
SkPathPriv.h as the go-between. If there's an existing convention
I overlooked, please let me know.

R=reed@google.com,djsollen@google.com
Bug: skia: 6898
Change-Id: I929d780c3b4a10e5cf78e7176a7516abf912508e
Reviewed-on: https://skia-review.googlesource.com/46560
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Cary Clark <caryclark@skia.org>
4 files changed