Move Dashing filterPath to a dashing utils file

Move Dashing filterPath to a dashing utils file

From inside GrContext, we have a need to create an SkPath an original path and
some dashing info. We do not have access to the original path effect so we need
a way to make the FilterPath function accessible outside of the effect. So I moved
the core filterPath code (and all need helper functions) out of SkDashPathEffect
and created a SkDashPath in utils to store these helper functions.

BUG=skia:
R=bsalomon@google.com, reed@google.com, rmistry@google.com

Author: egdaniel@google.com

Review URL: https://codereview.chromium.org/314623004