Introduce composed tree version of startOfDocument()

Introduce composed tree version of startOfDocument()

This patch introduces composed tree version of |startOfDocument()| by
templatizing original |startOfDocument()| 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://src.chromium.org/viewvc/blink?view=rev&revision=201979

[yosin_UTC9, 2015-09-09 08:27:06]

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

[yosin_UTC9, 2015-09-09 09:17:56]

PTAL

[hajimehoshi, 2015-09-09 10:54:57]

lgtm

[hajimehoshi, 2015-09-09 10:55:01]

The CQ bit was checked by hajimehoshi@chromium.org

[commit-bot: I haz the power, 2015-09-09 10:55:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324123003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324123003/1

[commit-bot: I haz the power, 2015-09-09 10:57:01]

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

[commit-bot: I haz the power, 2015-09-09 10:57:02]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)
  ios_rel_device_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)

[yosin_UTC9, 2015-09-09 11:06:29]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-09 11:06:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324123003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324123003/1

[commit-bot: I haz the power, 2015-09-09 11:08:38]

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

[commit-bot: I haz the power, 2015-09-09 11:08:38]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[yosin_UTC9, 2015-09-09 13:20:48]

The CQ bit was checked by yosin@chromium.org

[yosin_UTC9, 2015-09-09 13:20:48]

The patchset sent to the CQ was uploaded after l-g-t-m from
hajimehoshi@chromium.org
Link to the patchset: https://codereview.chromium.org/1324123003/#ps20001
(title: "2015-09-09T22:19:30 Rebase for VisibleUnitsTest.cpp conflict")

[commit-bot: I haz the power, 2015-09-09 13:20:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324123003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324123003/20001

[commit-bot: I haz the power, 2015-09-09 13:48:42]

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

[commit-bot: I haz the power, 2015-09-09 13:48:43]

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

[yosin_UTC9, 2015-09-09 13:54:38]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-09 13:54:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324123003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324123003/20001

[commit-bot: I haz the power, 2015-09-09 14:20:22]

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

[commit-bot: I haz the power, 2015-09-09 14:20:23]

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

[yosin_UTC9, 2015-09-09 14:24:19]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-09 14:24:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324123003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324123003/20001

[commit-bot: I haz the power, 2015-09-09 14:53:41]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201979