Move member function rootEditableElement() out from VisiblePosition class

Move member function rootEditableElement() out from VisiblePosition class

This patch moves member function |rootEditableElement()| out from
|VisiblePosition| class as global funciton |rootEditableElementOf()| to simplify
|VisiblePosition| for introducing composed tree version of |VisiblePosition|.

This patch is a preparation of making selection to handle granularity for web
component, http://crrev.com/1277863002

BUG=513568
TEST=n/a; no behavior changes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201479

[yosin_UTC9, 2015-08-31 08:46:35]

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

[yosin_UTC9, 2015-08-31 08:46:36]

PTAL

[yoichio, 2015-08-31 08:50:33]

Your patch missed VisiblePosition.h

[yosin_UTC9, 2015-08-31 08:56:20]

PTAL

[hajimehoshi, 2015-08-31 08:58:28]

lgtm

[yosin_UTC9, 2015-08-31 08:58:48]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-31 08:58:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1312213008/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1312213008/20001

[commit-bot: I haz the power, 2015-08-31 10:53:23]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201479