Move {next,previous}PositionOf() for VisiblePosition to VisibleUnits.cpp

Move {next,previous}PositionOf() for VisiblePosition to VisibleUnits.cpp

This patch moves |{next,previous}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=201500

[yosin_UTC9, 2015-08-31 09:48:30]

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

[yosin_UTC9, 2015-08-31 09:50:20]

yosin@chromium.org changed reviewers:
+ hajimehoshi@chromium.org
- hajimehoshi@chromium.orh

[yosin_UTC9, 2015-08-31 09:51:09]

PTAL

[hajimehoshi, 2015-08-31 10:33:22]

lgtm

[yosin_UTC9, 2015-08-31 10:41:33]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-31 10:41:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1307023005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1307023005/1

[commit-bot: I haz the power, 2015-08-31 10:43:37]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-31 10:43:38]

Try jobs failed on following builders:
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[yosin_UTC9, 2015-08-31 13:29:05]

The CQ bit was checked by yosin@chromium.org

[yosin_UTC9, 2015-08-31 13:29:06]

The patchset sent to the CQ was uploaded after l-g-t-m from
hajimehoshi@chromium.org
Link to the patchset: https://codereview.chromium.org/1307023005/#ps20001
(title: "2015-08-31T22:28:28 Rebase")

[commit-bot: I haz the power, 2015-08-31 13:29:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1307023005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1307023005/20001

[commit-bot: I haz the power, 2015-08-31 14:12:29]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-31 14:12:30]

Try jobs failed on following builders:
  win_chromium_x64_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_x64_...)

[yosin_UTC9, 2015-08-31 19:24:47]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-31 19:24:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1307023005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1307023005/20001

[commit-bot: I haz the power, 2015-08-31 20:01:49]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201500