A shadow host shouldn't receive an event, which must be stopped, from older shadow trees.

Spec bug: https://www.w3.org/Bugs/Public/show_bug.cgi?id=26595
Spec change: https://github.com/w3c/webcomponents/commit/15a5f294f4bb2f2cbd00e610e791ec9dc73527d0

This patch fixes the unintentional behavior in the following scenario:

Suppose a shadow host which hosts two shadow trees. Then an event which must be *stopped* [1] at shadow boundary is fired on node A in the older shadow tree.
The node A is distributed into the shadow insertion point in the younger shadow tree.

[1] http://w3c.github.io/webcomponents/spec/shadow/#events-that-are-always-stopped

The original behavior:
 A shadow host will receive the event.

This is not what I intended. In this case, the shadow host shouldn't receive the event while the younger shadow root as well as the older shadow root receive the event.

I am aware that it might be worth discussing whether nodes in the younger shadow trees should receive the event or not in this case. That could be discussed in the spec bug.

BUG=398527

Review URL: https://codereview.chromium.org/487713002

git-svn-id: svn://svn.chromium.org/blink/trunk@181242 bbb929c8-8fbe-4397-9dbb-9b2b20218538