Introduce composed tree version of nextPositionOf()

Introduce composed tree version of nextPositionOf()

This patch introduces composed tree version of |nextPositionOf()|
by templatizing original |nextPositionOf()| to work both DOM tree
position and composed tree position version for introducing composed tree
version of |VisibleSelection| class.

This patch also |honorEditingBoundaryAtOrAfter()| and
|skipToEndOfEditingBoundary()| used by |nextPositionOf()|.

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=201911

[yosin_UTC9, 2015-09-08 08:44:06]

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

[yosin_UTC9, 2015-09-08 08:44:54]

PTAL

[hajimehoshi, 2015-09-08 11:50:35]

lgtm

[yosin_UTC9, 2015-09-08 12:35:21]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-08 12:35:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1315443011/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1315443011/1

[commit-bot: I haz the power, 2015-09-08 13:25:33]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201911