Reland Properly set alpha type in webp decode.

Reland "Properly set alpha type in webp decode."

Also use the newer setConfig function.

Add a test to confirm that we set the alpha type properly.

Add some images with alpha for testing. (These images are also
beneficial for the compare_unpremul test, which was previously
not meaningful on 100% opaque images.)

All of the added images are in the public domain. They were
taken from https://developers.google.com/speed/webp/gallery2:

yellow_rose:
"Free Stock Photo in High Resolution - Yellow Rose 3 - Flowers"
Image Author: Jon Sullivan
This file is in the public domain.
http://www.public-domain-photos.com/free-stock-photos-4/flowers/yellow-rose-3.jpg

baby_tux:
"baby tux for my user page"
Image Author: Fizyplankton
This file is in the public domain.
http://www.minecraftwiki.net/images/8/85/Fizyplankton.png

NOTRY=true

TBR=halcanary@google.com

BUG=skia:2388
Committed: http://code.google.com/p/skia/source/detail?r=14360

[scroggo, 2014-04-24 18:47:50]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2014-04-24 18:48:31]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/scroggo@google.com/252423008/20001

[commit-bot: I haz the power, 2014-04-24 18:48:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-04-24 18:48:35]

Failed to request the patch to try. Please note that binary files are still
unsupported at the moment, this is being worked on.

Thanks for your patience.

Transient error: Can't process patch for file
resources/half-transparent-white-pixel.webp.
Binary file is empty. Maybe the file wasn't uploaded in the first place?

[scroggo, 2014-04-24 18:50:30]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2014-04-24 18:50:52]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/scroggo@google.com/252423008/20001

[commit-bot: I haz the power, 2014-04-24 18:50:55]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-04-24 18:50:55]

Failed to request the patch to try. Please note that binary files are still
unsupported at the moment, this is being worked on.

Thanks for your patience.

Transient error: Can't process patch for file
resources/half-transparent-white-pixel.webp.
Binary file is empty. Maybe the file wasn't uploaded in the first place?

[scroggo, 2014-04-24 18:54:07]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2014-04-24 18:54:56]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/scroggo@google.com/252423008/40001

[commit-bot: I haz the power, 2014-04-24 18:55:19]

Change committed as 14360