Move isVisuallyEquivalentCandidate() to VisibleUnits.{cpp,h} from Position.{cpp,h}

Move isVisuallyEquivalentCandidate() to VisibleUnits.{cpp,h} from Position.{cpp,h}

This patch moves |isVisuallyEquivalentCandidate()| to "VisibleUnits.{cpp,h} from
"Position.{cpp,h}" to consolidate |InlineBox| dependency to "VisibleUnits.cpp"
for ease of working them later.

This patch also gets rid of unused include directives from "Position.cpp".

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200971

[yosin_UTC9, 2015-08-21 06:01:08]

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

[yosin_UTC9, 2015-08-21 06:01:08]

PTAL

[yoichio, 2015-08-21 06:57:56]

lgtm

[yosin_UTC9, 2015-08-21 07:24:36]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-21 07:24:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1304063002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1304063002/1

[commit-bot: I haz the power, 2015-08-21 07:29:52]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200971