Move computeInlineBoxPosition() out from VisiblePosition

Move computeInlineBoxPosition() out from VisiblePosition

This patch moves |computeInlineBoxPosition()| out from |VisiblePosition| for
a preparation of moving |computeInlineBoxPosition()| out from |Position|
to make |Position| class simpler for improving code health.

BUG=518738
TEST=n/a; no behavior changes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200809

[yosin_UTC9, 2015-08-19 04:42:46]

The CQ bit was checked by yosin@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-19 04:42:56]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1302713002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1302713002/1

[yosin_UTC9, 2015-08-19 04:54:08]

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

[yosin_UTC9, 2015-08-19 04:54:09]

PTAL

[hajimehoshi, 2015-08-19 05:08:07]

lgtm

[commit-bot: I haz the power, 2015-08-19 08:01:10]

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

[commit-bot: I haz the power, 2015-08-19 08:01:11]

Dry run: This issue passed the CQ dry run.

[yosin_UTC9, 2015-08-19 08:33:17]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-19 08:33:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1302713002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1302713002/1

[commit-bot: I haz the power, 2015-08-19 08:37:37]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200809