Introduce previousPositionOf() for VisiblePosition

Introduce previousPositionOf() for VisiblePosition

This patch introduces |previousPositionOf()| function for |VisiblePosition| as
replacement of |VisiblePosition::previous()| to simplify |VisiblePosition| class
for ease of templataizing |VisiblePosition| class to introduce composed tree
version of |VisiblePosition|, since an implementation of
|VisiblePosition::previous()| can be implemented with public member functions of
|VisiblePosition| class.

This patch also moves |VisiblePosition::characterBefore()| into
"VisiblePosition.cpp" to avoid forward declaration of 
|previousPositionOf()|.

Note: http://crrev.com/1307803003 introduces |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=201382

[yosin_UTC9, 2015-08-28 08:10:27]

Patchset #2 (id:20001) has been deleted

[yosin_UTC9, 2015-08-28 08:10:53]

yosin@chromium.org changed reviewers:
+ tkent@chromium.org

[yosin_UTC9, 2015-08-28 08:12:23]

PTAL

[tkent, 2015-08-28 08:28:11]

lgtm

[yosin_UTC9, 2015-08-28 08:34:29]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-28 08:34:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1310043003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1310043003/1

[commit-bot: I haz the power, 2015-08-28 08:39:25]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201382