Make nextBoundary() to use lastPositionInNode() instead of RangeBoundaryPoint::setToEndOfNode()

Make nextBoundary() to use lastPositionInNode() instead of RangeBoundaryPoint::setToEndOfNode()

This patch changes |nextBoundary()| to use |lastPositionInNode()| instead of
|RangeBoundaryPoint::setToEndNode()| as preparation of making |nextBoundary()|
to work on composed tree, since |Position| class is templatzied to support
composed tree but |RangeBoundaryPoint| isn't.

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=201703

[yosin_UTC9, 2015-09-03 05:05:41]

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

[yosin_UTC9, 2015-09-03 05:05:49]

PTAL

[tkent, 2015-09-03 05:16:35]

rs lgtm

[yosin_UTC9, 2015-09-03 05:33:26]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-03 05:33:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1321133003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1321133003/1

[commit-bot: I haz the power, 2015-09-03 06:30:10]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201703