Introduce lineDirectionPointForBlockDirectionNavigationOf() for VisiblePosition

Introduce lineDirectionPointForBlockDirectionNavigationOf() for VisiblePosition

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

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=201387

[yosin_UTC9, 2015-08-28 08:36:51]

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

[yosin_UTC9, 2015-08-28 08:59:53]

PTAL

[hajimehoshi, 2015-08-28 09:10:29]

lgtm

[yosin_UTC9, 2015-08-28 09:10:52]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-28 09:11:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324463002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324463002/1

[commit-bot: I haz the power, 2015-08-28 09:44:02]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201387