Introduce composed tree version of canonicalPosition() in VisiblePosition

Introduce composed tree version of canonicalPosition() in VisiblePosition

This patch introduces composed tree version of |canonicalPosition()| along with
composed tree version of functions called from |canonicalPosition()| for
a preparation of making |VisiblePostion| to work with composed tree.

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/1106433002.

BUG=275851
TEST=webkit_unit_tests --gtest_filter=VisiblePosition*

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197649

[yosin_UTC9, 2015-06-23 07:46:37]

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

[yosin_UTC9, 2015-06-23 07:46:48]

PTAL

[tkent, 2015-06-23 07:56:06]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-06-23 07:56:07]

lgtm

https://codereview.chromium.org/1201153002/diff/1/Source/core/editing/Visible...
File Source/core/editing/VisiblePositionTest.cpp (right):

https://codereview.chromium.org/1201153002/diff/1/Source/core/editing/Visible...
Source/core/editing/VisiblePositionTest.cpp:52:
PassRefPtrWillBeRawPtr<ShadowRoot>
VisiblePositionTest::createShadowRootForElementWithIDAndSetInnerHTML(TreeScope&
scope, const char* hostElementID, const char* shadowRootContent)
I saw this code multiple times.  Can you merge them by a cleanup CL?

[commit-bot: I haz the power, 2015-06-23 07:56:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1201153002/1

[commit-bot: I haz the power, 2015-06-23 10:22:05]

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

[commit-bot: I haz the power, 2015-06-23 10:22:06]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/67838)

[hajimehoshi, 2015-06-23 10:30:33]

The CQ bit was checked by hajimehoshi@chromium.org

[commit-bot: I haz the power, 2015-06-23 10:30:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1201153002/1

[commit-bot: I haz the power, 2015-06-23 12:56:49]

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

[commit-bot: I haz the power, 2015-06-23 12:56:50]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/67858)

[yosin_UTC9, 2015-06-23 13:22:34]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-06-23 13:23:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1201153002/1

[commit-bot: I haz the power, 2015-06-23 14:22:04]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197649