DevTools: do not throw error to console on AddNewRule

DevTools: do not throw error to console on AddNewRule

Obviously, an empty text is a valid text for a stylesheet. No need to
throw.

BUG=505358
R=pfeldman

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=198010

[lushnikov, 2015-06-29 12:47:31]

ptal

[pfeldman, 2015-06-29 12:48:59]

lgtm

[lushnikov, 2015-06-29 12:49:20]

The CQ bit was checked by lushnikov@chromium.org

[commit-bot: I haz the power, 2015-06-29 12:49:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1212643005/1

[commit-bot: I haz the power, 2015-06-29 15:34:19]

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

[commit-bot: I haz the power, 2015-06-29 15:34:20]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/68618)

[lushnikov, 2015-06-29 17:06:01]

The CQ bit was checked by lushnikov@chromium.org

[commit-bot: I haz the power, 2015-06-29 17:06:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1212643005/1

[commit-bot: I haz the power, 2015-06-29 18:45:42]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=198010