Introduce ComposedTreeTraversal::previousPostOrder()

Introduce ComposedTreeTraversal::previousPostOrder()

This patch introduces |ComposedTreeTraversal::previousPostOrder()| which is
composed tree version of |NodeTraversal::previousPostOrder()| as a preparation
of implementing composed tree version of |startOfParagraph()|.

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=201965

[yosin_UTC9, 2015-09-09 08:03:59]

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

[yosin_UTC9, 2015-09-09 08:04:12]

PTAL

[tkent, 2015-09-09 08:38:17]

rs lgtm.

I confirmed the implementation was equivalent to
NodeTraversal::previousPostOrder.

[yosin_UTC9, 2015-09-09 08:48:48]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-09 08:49:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1308063008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1308063008/1

[commit-bot: I haz the power, 2015-09-09 09:56:48]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201965