The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4902d6ccd120be91c417b891b13d6fdb0b4e8e4c

commit 4902d6ccd120be91c417b891b13d6fdb0b4e8e4c
Author: hayato <hayato@chromium.org>
Date: Fri Apr 08 05:59:35 2016

Update the behavior of ShadowRoot.activeElement to match the latest spec

The spec change is:
https://github.com/w3c/webcomponents/commit/15a203c8393aef0df7223ab1d43406aa11a7e71e

- v1 shadow tree should have the new behavior.
- v0 shadow tree will not change the behavior for a compatibility.

BUG= 601303 

Review URL: https://codereview.chromium.org/1870583002

Cr-Commit-Position: refs/heads/master@{#386002}

[add] https://crrev.com/4902d6ccd120be91c417b891b13d6fdb0b4e8e4c/third_party/WebKit/LayoutTests/shadow-dom/active-element.html
[modify] https://crrev.com/4902d6ccd120be91c417b891b13d6fdb0b4e8e4c/third_party/WebKit/Source/core/dom/Node.cpp
[modify] https://crrev.com/4902d6ccd120be91c417b891b13d6fdb0b4e8e4c/third_party/WebKit/Source/core/dom/Node.h
[modify] https://crrev.com/4902d6ccd120be91c417b891b13d6fdb0b4e8e4c/third_party/WebKit/Source/core/dom/TreeScope.cpp