Make PositionAlgorithm<Strategy>::next() to use Strategy::index()

Make PositionAlgorithm<Strategy>::next() to use Strategy::index()

This patch changes |PositionAlgorithm<Strategy>::next()| to use
|Strategy::index(Node&)| instead of |Node::nodexIndex()| to compute offset
of node in parent node with DOM tree or composed tree base on |Strategy|
template parameter for correct behavior in |PositionInComposedTree|.

This patch is a preparation of templatizing |VisiblePosition|.

BUG=513568
TEST=PositionTest.NextNodeIndex

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=199920

[yosin_UTC9, 2015-08-03 07:54:28]

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

[yosin_UTC9, 2015-08-03 09:05:21]

PTAL

[hajimehoshi, 2015-08-03 09:52:25]

lgtm

[hajimehoshi, 2015-08-03 09:52:29]

The CQ bit was checked by hajimehoshi@chromium.org

[commit-bot: I haz the power, 2015-08-03 09:52:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1263033003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1263033003/1

[commit-bot: I haz the power, 2015-08-03 09:57:28]

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

[commit-bot: I haz the power, 2015-08-03 09:57:28]

Try jobs failed on following builders:
  blink_presubmit on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/blink_presubmit/builds/3...)

[tkent, 2015-08-03 22:51:39]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-08-03 22:51:39]

lgtm

[commit-bot: I haz the power, 2015-08-03 22:52:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1263033003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1263033003/1

[commit-bot: I haz the power, 2015-08-03 22:55:42]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=199920