Reland of ix zero-length tangent

Reland of ix zero-length tangent (patchset #1 id:1 of https://codereview.chromium.org/1312243002/ )

Reason for revert:
Layout suppression has landed, and verified that Skia gm test changes are correct.

Original issue's description:
> 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
> 
> Committed: https://skia.googlesource.com/skia/+/91298b47c547b2ab4697038c04685af957bd1416

TBR=reed@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:4191
Committed: https://skia.googlesource.com/skia/+/45398dff710c6cdadbc3c686faa7bf692febd222

[caryclark, 2015-08-25 20:18:52]

Created Reland of ix zero-length tangent

[commit-bot: I haz the power, 2015-08-25 20:18:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1320473002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1320473002/1

[commit-bot: I haz the power, 2015-08-25 20:19:11]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/45398dff710c6cdadbc3c686faa7bf692febd222