Move honorEditingBoundaryAtOr{Before,After}() for VisiblePosition to VisibleUnits.cpp

Move honorEditingBoundaryAtOr{Before,After}() for VisiblePosition to VisibleUnits.cpp

This patch moves |honorEditingBoundaryAtOr{Before,After}()| 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=201622

[yosin_UTC9, 2015-09-02 05:30:47]

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

[yosin_UTC9, 2015-09-02 05:30:54]

PTAL

[hajimehoshi, 2015-09-02 06:48:24]

lgtm

[yosin_UTC9, 2015-09-02 06:49:37]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-02 06:50:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1303173010/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1303173010/1

[commit-bot: I haz the power, 2015-09-02 06:55:02]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201622