Implement ShadowRoot.delegatesFocus 3/4 (Slide focus on focus() method)

Implement ShadowRoot.delegatesFocus 3/4 (Slide focus on focus() method)

This Implements sliding focus from shadow host to
inner element of its shadow root on focus().

When delegatesFocus is set to true for a shadow root
and its host gets focus() from script, the shadow host
delegates focus to its internal focusable element.

This depends on previous CLs.
https://codereview.chromium.org/1174893002/
https://codereview.chromium.org/1174933002/

Remaining CL:
4/4 Match CSS :focus for shadow host

Design doc:
https://docs.google.com/document/d/1k93Ez6yNSyWQDtGjdJJqTBPmljk9l2WS3JTe5OHHB50/edit

BUG=496005
TEST=fast/dom/shadow/focus-method-with-delegatesFocus.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196927

[kochi, 2015-06-10 11:13:20]

The CQ bit was checked by kochi@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-10 11:13:30]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1177673004/20001

[commit-bot: I haz the power, 2015-06-10 13:15:06]

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

[commit-bot: I haz the power, 2015-06-10 13:15:07]

Dry run: 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/66025)

[kochi, 2015-06-11 02:06:55]

kochi@chromium.org changed reviewers:
+ hayato@chromium.org

[kochi, 2015-06-11 04:05:30]

PTAL

[hayato, 2015-06-11 04:27:19]

lgtm

[kochi, 2015-06-11 07:15:46]

The CQ bit was checked by kochi@chromium.org

[commit-bot: I haz the power, 2015-06-11 07:15:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1177673004/40001

[commit-bot: I haz the power, 2015-06-11 08:31:58]

Committed patchset #3 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196927