Revert of fix zero-length tangent (patchset #2 id:20001 of https://codereview.chromium.org/1311273002/ )

Reason for revert:
causes layout test to draw differently -- new drawing is more correct. Reverting until layout test ignore is landed.

Original issue's description:
> fix zero-length tangent
>
> If the end point and the control point are the same, computing
> the tangent will result in (0, 0). In this case, use the prior
> control point instead.
>
> R=reed@google.com
>
> BUG=skia:4191
>
> Committed: https://skia.googlesource.com/skia/+/7544124fb8ee744f68f549a353f8a9163cd7432d

TBR=reed@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:4191

Review URL: https://codereview.chromium.org/1312243002
3 files changed