Fix casting bug in GrArithmeticXP

Fix casting bug in GrArithmeticXP

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/f16201250315bd807e06547f1c412fcdc5c8805a

[egdaniel, 2015-02-04 17:03:49]

egdaniel@google.com changed reviewers:
+ joshualitt@chromium.org

[egdaniel, 2015-02-04 17:03:49]

ptal

[egdaniel, 2015-02-04 17:04:01]

The CQ bit was checked by egdaniel@google.com

[commit-bot: I haz the power, 2015-02-04 17:05:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/858373006/1

[commit-bot: I haz the power, 2015-02-04 17:05:01]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-02-04 23:05 UTC

[joshualitt, 2015-02-04 17:21:24]

On 2015/02/04 17:05:01, I haz the power (commit-bot) wrote:
> Note for Reviewers:
> The CQ is waiting for an approval.
> If you believe that the CL is not ready yet, or if you would like to L-G-T-M
> with comments then please uncheck the CQ checkbox.
> 
> Waiting for LGTM from valid reviewer(s) till 2015-02-04 23:05 UTC

lgtm

[commit-bot: I haz the power, 2015-02-04 17:22:37]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/f16201250315bd807e06547f1c412fcdc5c8805a