"*" in a shadow tree should not match its shadow host.

Shadow DOM Styling spec:
http://dev.w3.org/csswg/shadow-styling/#host-element

The host element is not selectable by any means except for the :host pseudo-class.
In particular, the host element isn’t matched by the * selector either.

BUG=351248
TEST=fast/dom/shadow/shadow-tree-styles-select-host.html

Review URL: https://codereview.chromium.org/194853002

git-svn-id: svn://svn.chromium.org/blink/trunk@168984 bbb929c8-8fbe-4397-9dbb-9b2b20218538