Introduce positionForPoint() in LocalFrame class as replacement of visiblePositionForPoint()

Introduce positionForPoint() in LocalFrame class as replacement of visiblePositionForPoint()

This patch introduces |positionForPoint()| in |LocalFrame| class as replacement
of |visiblePositionForPoint()| to reduce doing visible position
canonicalization for increasing chance to delay canonicalization with changing
|setPosition()| in |DragCaretController| and |VisibleSelection| constructor to
take |PositionWithAffinity|.

Note: Both of |VisbilePosition| constructor and |VisibleSelection| constructor
do visible position canaonicalization.

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=201110

[yosin_UTC9, 2015-08-25 07:20:57]

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

[yosin_UTC9, 2015-08-25 07:21:37]

PTAL

[tkent, 2015-08-25 07:42:40]

lgtm

[yosin_UTC9, 2015-08-25 08:06:15]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-25 08:06:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1310323004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1310323004/1

[commit-bot: I haz the power, 2015-08-25 08:11:27]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201110