Implement ShadowRoot.delegatesFocus 2/4 (slide focus on mouse click)

Implement ShadowRoot.delegatesFocus 2/4 (slide focus on mouse click)

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

When delegatesFocus is set to true for a shadow root, its shadow host
delegates mouse click-initiated focus to its internal focusable element.

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

Remaining implementations will follow:
3/4 Slide focus on focus() method
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-slide-on-shadow-host.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196925

[kochi, 2015-06-10 11:13:25]

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

[commit-bot: I haz the power, 2015-06-10 11:13:29]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174933002/1

[commit-bot: I haz the power, 2015-06-10 12:33:05]

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

[commit-bot: I haz the power, 2015-06-10 12:33:06]

Dry run: This issue passed the CQ dry run.

[kochi, 2015-06-11 02:05:11]

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

[kochi, 2015-06-11 04:05:12]

PTAL

[hayato, 2015-06-11 04:26:13]

lgtm

[kochi, 2015-06-11 07:14:33]

The CQ bit was checked by kochi@chromium.org

[commit-bot: I haz the power, 2015-06-11 07:14:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174933002/20001

[commit-bot: I haz the power, 2015-06-11 08:19:44]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196925