Fix build warnings in SkAlphaThresholdFilter

Fix build warnings in SkAlphaThresholdFilter

BUG=None

Committed: http://code.google.com/p/skia/source/detail?r=12937

[Zachary Kuznia, 2014-01-07 02:00:48]

Hi Brian,

Please take a look at this.  It should fix the broken builds on the waterfall.

Thanks,
-Zach

[bsalomon, 2014-01-07 15:20:54]

lgtm

[commit-bot: I haz the power, 2014-01-07 15:21:01]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/zork@chromium.org/101763010/210001

[mtklein, 2014-01-07 15:27:06]

On 2014/01/07 15:21:01, I haz the power (commit-bot) wrote:
> CQ is trying da patch. Follow status at
> https://skia-tree-status.appspot.com/cq/zork%40chromium.org/101763010/210001

While we're looking at this, I've noticed the new GM is failing in some
serialization modes:

$ out/Release/dm
...
0 tasks left, 3 failed
Failures:
  imagealphathreshold_8888_cross_process_pipe
  imagealphathreshold_8888_cross_process_shared_address_space_pipe
  imagealphathreshold_8888_serialize

That is, it's not drawing the same when the commands are serialized and
deserialized (both picture and pipe).  I figure fixing the _serialize case will
fix the others.

[commit-bot: I haz the power, 2014-01-07 16:04:10]

Change committed as 12937

[mtklein, 2014-01-07 17:02:27]

On 2014/01/07 16:04:10, I haz the power (commit-bot) wrote:
> Change committed as 12937

GM is also failing on the bots for the same reason.

[bsalomon, 2014-01-07 17:05:12]

On 2014/01/07 17:02:27, mtklein wrote:
> On 2014/01/07 16:04:10, I haz the power (commit-bot) wrote:
> > Change committed as 12937
> 
> GM is also failing on the bots for the same reason.

I'll exclude it until we have a fix.