Implement nameFromLabelElement to see if a control is associated with a label.

Implement nameFromLabelElement to see if a control is associated with a label.

In a few places in the code, we have an HTML control and a label element,
and we want to know if that label is providing the name for that control.

It seems clean to call axObject.name() and check the returned nameFrom,
but computing the whole name has side effects (creating AXObjects of
descendants) so it caused a crash. Instead, nameFromLabelElement is simpler
and computes just what's needed.

This unfortunately duplicates a small amount of logic. I added comments to
hopefully avoid introducing more errors in the future.

BUG=558689
Committed: https://crrev.com/183bda8ea817bee6edd0589d02c611fc09fb9cee
Cr-Commit-Position: refs/heads/master@{#361395}

[dmazzoni, 2015-11-23 22:16:43]

dmazzoni@chromium.org changed reviewers:
+ aboxhall@chromium.org

[aboxhall, 2015-11-23 22:45:39]

lgtm

Yeah, this seems like the only sensible way to do this.

[dmazzoni, 2015-11-23 23:15:13]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2015-11-23 23:17:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1467423002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1467423002/1

[commit-bot: I haz the power, 2015-11-24 00:42:30]

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

[commit-bot: I haz the power, 2015-11-24 00:42:31]

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_...)

[dmazzoni, 2015-11-24 18:03:49]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2015-11-24 18:05:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1467423002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1467423002/1

[commit-bot: I haz the power, 2015-11-24 18:37:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-24 18:39:11]

Patchset 1 (id:??) landed as
https://crrev.com/183bda8ea817bee6edd0589d02c611fc09fb9cee
Cr-Commit-Position: refs/heads/master@{#361395}