Move a member function atEditingBoundary() out from PositionAlgorithm<Strategy> template class

Move a member function atEditingBoundary() out from PositionAlgorithm<Strategy> template class

This patch moves a member function |atEditingBoundary()| in
|PositionAlgorithm<Strategy>| template class as static function inside
"Position.cpp" to simplify |PositionAlgorithm| class for improving code halth.

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200814

[yosin_UTC9, 2015-08-19 05:58:43]

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

[yosin_UTC9, 2015-08-19 05:58:44]

PTAL

[yoichio, 2015-08-19 06:03:49]

lgtm

[yosin_UTC9, 2015-08-19 07:37:21]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-19 07:37:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1288553007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1288553007/1

[commit-bot: I haz the power, 2015-08-19 09:27:41]

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

[commit-bot: I haz the power, 2015-08-19 09:27:42]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[yosin_UTC9, 2015-08-19 09:28:51]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-19 09:29:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1288553007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1288553007/1

[commit-bot: I haz the power, 2015-08-19 10:36:27]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200814