Prevent division by 0 in set_luminance shader function.

Prevent division by 0 in set_luminance shader function.

The solution has been approved in skia: https://codereview.chromium.org/666043003/

BUG=

Committed: https://crrev.com/fd7bcf3a5419a86e4903f43e3dd016e33929082e
Cr-Commit-Position: refs/heads/master@{#301333}
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

[rosca, 2014-10-24 09:57:33]

rosca@adobe.com changed reviewers:
+ enne@chromium.org

[rosca, 2014-10-24 09:57:33]

Please review.

[enne (OOO), 2014-10-24 22:57:06]

lgtm

[rosca, 2014-10-25 06:49:23]

The CQ bit was checked by rosca@adobe.com

[commit-bot: I haz the power, 2014-10-25 06:49:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/639773010/1

[commit-bot: I haz the power, 2014-10-25 07:42:02]

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

[commit-bot: I haz the power, 2014-10-25 07:42:03]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[rosca, 2014-10-27 09:19:32]

The CQ bit was checked by rosca@adobe.com

[commit-bot: I haz the power, 2014-10-27 09:19:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/639773010/20001

[commit-bot: I haz the power, 2014-10-27 10:07:28]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-10-27 10:08:06]

Patchset 2 (id:??) landed as
https://crrev.com/fd7bcf3a5419a86e4903f43e3dd016e33929082e
Cr-Commit-Position: refs/heads/master@{#301333}

[rosca, 2015-06-07 21:41:16]

Patchset #3 (id:40001) has been deleted