Move {left,right}PositionOf() for VisiblePosition to VisibleUnits.cpp

Move {left,right}PositionOf() for VisiblePosition to VisibleUnits.cpp

This patch moves |{left,right}PositionOf()| for |VisiblePosition| to
"VisibleUnits.cpp" form "VisiblePosition.cpp" to make "VisiblePosition.cpp"
to simpler for improving code health and ease of templatizing |VisiblePosition|
class to introduce composed tree version.

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=201595

[yosin_UTC9, 2015-09-01 13:07:15]

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

[yosin_UTC9, 2015-09-01 13:07:24]

PTAL

[yoichio, 2015-09-02 00:54:47]

lgtm

[yosin_UTC9, 2015-09-02 00:55:28]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-02 00:55:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1310953005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1310953005/1

[commit-bot: I haz the power, 2015-09-02 01:53:32]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201595