Introduce composed tree version of startOfWord()

Introduce composed tree version of startOfWord()

This patch introduces composed tree version of |startOfWord()| by
templatizing original |startOfWord()| to work both DOM tree position and
composed tree position version for introducing composed tree version of
|VisibleSelection| class.

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://crrev.com/de9d06a43d8514338769e6e8d013eb03eb4e76f2
git-svn-id: svn://svn.chromium.org/blink/trunk@202182 bbb929c8-8fbe-4397-9dbb-9b2b20218538

[yosin_UTC9, 2015-09-14 03:33:26]

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

[yosin_UTC9, 2015-09-14 03:33:27]

PTAL

[yoichio, 2015-09-14 03:36:08]

lgtm

[yosin_UTC9, 2015-09-14 03:53:28]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-14 03:53:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1338953002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1338953002/1

[commit-bot: I haz the power, 2015-09-14 04:45:31]

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

[commit-bot: I haz the power, 2015-09-14 04:45:32]

Try jobs failed on following builders:
  linux_chromium_gn_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[yosin_UTC9, 2015-09-14 05:42:35]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-14 05:42:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1338953002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1338953002/1

[commit-bot: I haz the power, 2015-09-14 06:05:02]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=202182

[commit-bot: I haz the power, 2015-09-23 12:30:00]

Patchset 1 (id:??) landed as
https://crrev.com/de9d06a43d8514338769e6e8d013eb03eb4e76f2