Move parentEditingBoundary() to VisibileUnits.cpp from Position.cpp.

Move parentEditingBoundary() to VisibileUnits.cpp from Position.cpp.

This patch moves |parentEditingBoundary()| to "VisibleUnits.{cpp,h}" from
"Position.{cpp,h}" with |nonShadowBoundaryParentNode()| which is used only by
|parentEditingBoundary()| to cleanup "Position.{cpp,h}" for improving code
health, since |parentEditingBoundary()| is used only in "VisibleUnits.cpp".

BUG=518738
TEST=n/a; no behavior changes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200907

[yosin_UTC9, 2015-08-20 09:21:28]

yosin@chromium.org changed reviewers:
+ hajimehoshi@chromium.org, yoichio@chromium.org

[yosin_UTC9, 2015-08-20 09:21:29]

PTAL

[hajimehoshi, 2015-08-20 11:07:38]

lgtm

[yosin_UTC9, 2015-08-20 11:21:48]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-20 11:22:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1288063008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1288063008/1

[commit-bot: I haz the power, 2015-08-20 11:26:54]

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

[commit-bot: I haz the power, 2015-08-20 11:26:54]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[yosin_UTC9, 2015-08-20 13:52:13]

The CQ bit was checked by yosin@chromium.org

[yosin_UTC9, 2015-08-20 13:52:13]

The patchset sent to the CQ was uploaded after l-g-t-m from
hajimehoshi@chromium.org
Link to the patchset: https://codereview.chromium.org/1288063008/#ps20001
(title: "2015-08-20T22:51:43 Rebase")

[commit-bot: I haz the power, 2015-08-20 13:52:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1288063008/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1288063008/20001

[commit-bot: I haz the power, 2015-08-20 15:52:48]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200907