Get rid of unused member function rootEditableElement() in PositionAlgorithm<Strategy> template clas

Get rid of unused member function rootEditableElement() in PositionAlgorithm<Strategy> template class

This patch gets rid of unused member function |rootEditableElement()| in
|PositionAlgorithm<Strategy>| template class to simplify source code for
improving code health.

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200805

[yosin_UTC9, 2015-08-19 05:05:20]

The CQ bit was checked by yosin@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-19 05:05:42]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1303673002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1303673002/1

[yosin_UTC9, 2015-08-19 05:05:47]

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

[yosin_UTC9, 2015-08-19 05:05:58]

PTAL

[yoichio, 2015-08-19 06:07:51]

lgtm

[commit-bot: I haz the power, 2015-08-19 06:49:06]

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

[commit-bot: I haz the power, 2015-08-19 06:49:07]

Dry run: This issue passed the CQ dry run.

[yosin_UTC9, 2015-08-19 07:23:58]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-19 07:24:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1303673002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1303673002/1

[commit-bot: I haz the power, 2015-08-19 07:28:34]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200805