add one-channel output support to webp encoder

add one-channel output support to webp encoder

BUG=

Committed: http://code.google.com/p/skia/source/detail?r=14340

[humper, 2014-04-23 19:26:34]

PTAL

[hal.canary, 2014-04-23 19:35:57]

On 2014/04/23 19:26:34, humper wrote:
> PTAL

Do we test this codepath anywhere?  How did you test it?

[humper, 2014-04-23 20:02:52]

On 2014/04/23 19:35:57, Hal Canary wrote:
> On 2014/04/23 19:26:34, humper wrote:
> > PTAL
> 
> Do we test this codepath anywhere?  How did you test it?

Sorry, I tested it by using it in another CL; then I cut it out from that CL and
put it in a separate one so they didn't get conflated.

I don't have a standalone test for it, no.  It does work, I promise :P

[hal.canary, 2014-04-23 20:08:04]

lgtm

[humper, 2014-04-23 20:58:32]

The CQ bit was checked by humper@google.com

[commit-bot: I haz the power, 2014-04-23 20:58:56]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/humper@google.com/249853003/1

[commit-bot: I haz the power, 2014-04-23 21:55:33]

Change committed as 14340