Introduce EditingAlgorithm<Traversal> class

Introduce EditingAlgorithm<Traversal> class

This patch introduces |EditingAlgorithm<Traversal>| template class to use
same algorithm on both DOM tree and composed tree for avoiding code duplication.


This patch is result of collaboration work with hajimehoshi@chromium.org
for selection of web components. After this patch, we are going to
implement another strategies for the composed tree. We've already
prepared that at http://crrev.com/1009603011.

BUG=275851
TEST=n/a; No behavior changes.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=193766

[yosin_UTC9, 2015-04-15 04:25:33]

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

[yosin_UTC9, 2015-04-15 06:19:18]

PTAL

[tkent, 2015-04-15 06:33:58]

lgtm

[yosin_UTC9, 2015-04-15 07:52:17]

The CQ bit was checked by yosin@chromium.org

[yosin_UTC9, 2015-04-15 07:52:17]

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/1085163002/#ps20001
(title: "2015-04-15T16:50:42 Rebase for Position.h updates")

[commit-bot: I haz the power, 2015-04-15 07:52:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1085163002/20001

[commit-bot: I haz the power, 2015-04-15 09:30:27]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=193766