Make toPositionInComposedTree() with non-distributed node to return non-null position

Make toPositionInComposedTree() with non-distributed node to return non-null position

This patch makes |toPositionInComposedTree()| with a position at non-distributed
node to return a position after children in shadow host rather than null
position.

This patch is a preparation of making selection to handle granularity for web
component, http://crrev.com/1363873004

BUG=513568
TEST=PositionTest.ToPositionInComposedTreeWithNotDistributed

Committed: https://crrev.com/18f60f0b43d791ca4b2c90cbcf7f49dfde1cc46c
Cr-Commit-Position: refs/heads/master@{#350741}

[yosin_UTC9, 2015-09-24 12:32:06]

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

[yosin_UTC9, 2015-09-24 12:32:06]

PTAL

[tkent, 2015-09-25 01:02:52]

rs lgtm

[yosin_UTC9, 2015-09-25 01:17:54]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-25 01:19:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1362283003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1362283003/1

[commit-bot: I haz the power, 2015-09-25 01:40:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-25 01:40:55]

Patchset 1 (id:??) landed as
https://crrev.com/18f60f0b43d791ca4b2c90cbcf7f49dfde1cc46c
Cr-Commit-Position: refs/heads/master@{#350741}