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

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

This patch moves |character{After,Before}()| 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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201519

[yosin_UTC9, 2015-09-01 03:03:26]

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

[yosin_UTC9, 2015-09-01 03:03:34]

PTAL

[yoichio, 2015-09-01 03:35:50]

lgtm

[yosin_UTC9, 2015-09-01 04:06:42]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-01 04:06:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1320233003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1320233003/1

[commit-bot: I haz the power, 2015-09-01 04:38:19]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201519