Generate correct error from WEBGL_draw_buffers extension.

Generate correct error from WEBGL_draw_buffers extension.

Should produce INVALID_OPERATION, not INVALID_VALUE, when number of
buffers != 1 when default back buffer is bound.

BUG=559359
Committed: https://crrev.com/a423527b68fc52203b4b5ba3b2de2f2dc9842f66
Cr-Commit-Position: refs/heads/master@{#361401}

[Ken Russell (switch to Gerrit), 2015-11-24 02:11:18]

kbr@chromium.org changed reviewers:
+ bajones@chromium.org, zmo@chromium.org

[Ken Russell (switch to Gerrit), 2015-11-24 02:11:18]

PTAL

[Ken Russell (switch to Gerrit), 2015-11-24 02:11:22]

The CQ bit was checked by kbr@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-11-24 02:13:28]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1473103002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1473103002/1

[commit-bot: I haz the power, 2015-11-24 04:30:06]

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

[commit-bot: I haz the power, 2015-11-24 04:30:07]

Dry run: Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[bajones, 2015-11-24 16:37:56]

On 2015/11/24 04:30:07, commit-bot: I haz the power wrote:
> Dry run: Try jobs failed on following builders:
>   linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
>
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

LGTM

[Ken Russell (switch to Gerrit), 2015-11-24 18:15:54]

The CQ bit was checked by kbr@chromium.org

[commit-bot: I haz the power, 2015-11-24 18:17:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1473103002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1473103002/1

[commit-bot: I haz the power, 2015-11-24 18:49:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-24 18:50:21]

Patchset 1 (id:??) landed as
https://crrev.com/a423527b68fc52203b4b5ba3b2de2f2dc9842f66
Cr-Commit-Position: refs/heads/master@{#361401}