Introduce composed tree version of previousPositionOf()

Introduce composed tree version of previousPositionOf()

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

This patch also templatizes |skipToStartOfEditingBoundary()| used by
|previousPositionOf()|.

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=201963

[yosin_UTC9, 2015-09-09 06:39:40]

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

[yosin_UTC9, 2015-09-09 06:39:41]

PTAL

[yoichio, 2015-09-09 07:00:21]

lgtm

[yosin_UTC9, 2015-09-09 07:09:38]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-09 07:09:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1312473017/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1312473017/1

[commit-bot: I haz the power, 2015-09-09 09:45:40]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201963