Make previousBoundary() not to use Range object

Make previousBoundary() not to use Range object

This patch changes |previousBoundary()| to use |Position::afterNode()| instead
of |Range| object to avoid registering temporary |Range| object to |Document|
and as a preparation of |previousBoundary()| to work on composed tree by
templatization.

This patch is a preparation of making selection to handle granularity for web
component, http://crrev.com/1277863002

BUG=388681, 513568
TEST=n/a; no behavior changes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201708

[yosin_UTC9, 2015-09-03 06:10:13]

yosin@chromium.org changed reviewers:
+ tkent@chromium.org

[yosin_UTC9, 2015-09-03 06:10:13]

PTAL

[tkent, 2015-09-03 06:14:52]

rs-lgtm

[yosin_UTC9, 2015-09-03 06:35:36]

The CQ bit was checked by yosin@chromium.org

[yosin_UTC9, 2015-09-03 06:35:36]

The patchset sent to the CQ was uploaded after l-g-t-m from tkent@chromium.org
Link to the patchset: https://codereview.chromium.org/1307233006/#ps20001
(title: "2015-09-03T15:34:51 Remove redundant #include ExceptionState.h"")

[commit-bot: I haz the power, 2015-09-03 06:35:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1307233006/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1307233006/20001

[commit-bot: I haz the power, 2015-09-03 07:44:46]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201708