Move Position::isRenderedCharacter() for Text node to LayoutText

Move Position::isRenderedCharacter() for Text node to LayoutText

This patch moves an implementation of |Position::isRenderedCharacter()| for
|Text| node to |LayoutText| to reduce usage of |InlineBox| in |Position| class
for improving code health and helping identify layout API for editing.

This patch is a preparation of handling of :first-letter pseudo element in
selection.

Following patch will move other |InlineBox| references in editing/ to layout/.

BUG=174349, 518738
TEST=n/a; no behavior changes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200407

[yosin_UTC9, 2015-08-12 08:26:46]

yosin@chromium.org changed reviewers:
+ leviw@chromium.org, ojan@chromium.org, tkent@chromium.org,
yoichio@chromium.org

[yosin_UTC9, 2015-08-12 08:27:51]

PTAL

[yoichio, 2015-08-12 08:33:19]

lgtm

[leviw_travelin_and_unemployed, 2015-08-12 18:13:19]

The CQ bit was checked by leviw@chromium.org

[leviw_travelin_and_unemployed, 2015-08-12 18:13:20]

lgtm

[commit-bot: I haz the power, 2015-08-12 18:13:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1285123002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1285123002/1

[commit-bot: I haz the power, 2015-08-12 18:16:42]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200407