WebUI: Fix radio and checkbox behavior when text selected.

WebUI: Fix radio and checkbox behavior when text selected.

Previously, when text or a link was selected, clicking on the label portion
of a radio or checkbox would not work.

This fixes that by applying the -webkit-user-select: none; to the whole label
instead of just the text portion of it. Why this works? Who knows.

But it does work.

BUG=497669
TEST=MANUAL

Committed: https://crrev.com/bc37351bca408ebd2006823b46fb0ca691afd74e
Cr-Commit-Position: refs/heads/master@{#336695}

[tommycli, 2015-06-24 23:24:42]

tommycli@chromium.org changed reviewers:
+ dbeam@chromium.org

[tommycli, 2015-06-24 23:25:05]

dbeam: PTAL

[tommycli, 2015-06-24 23:41:45]

tommycli@chromium.org changed reviewers:
+ estade@chromium.org
- dbeam@chromium.org

[tommycli, 2015-06-24 23:43:44]

estade: PTAL

See bug.

We discovered -webkit-user-select misbehaves when only applied to a sub-element
of a label instead of the whole label. No idea why. Probably a Blink bug.

This patch moves -webkit-user-select to the parent <label> instead of the inner
<span>. This fixes the problem.

But any thoughts on just removing -webkit-user-select entirely? It would also
solve the problem, and just allow users to select radio and checkbox text.

[tommycli, 2015-06-26 17:38:14]

estade: ping

[tommycli, 2015-06-29 19:45:52]

On 2015/06/26 17:38:14, tommycli wrote:
> estade: ping

hi estade: ping

[Evan Stade, 2015-06-29 23:16:52]

lgtm

[tommycli, 2015-06-29 23:17:14]

The CQ bit was checked by tommycli@chromium.org

[commit-bot: I haz the power, 2015-06-29 23:18:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1203183003/1

[commit-bot: I haz the power, 2015-06-30 00:29:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-30 00:30:52]

Patchset 1 (id:??) landed as
https://crrev.com/bc37351bca408ebd2006823b46fb0ca691afd74e
Cr-Commit-Position: refs/heads/master@{#336695}