SelectorChecker should check if previousElement is null for ::shadow.

We cannot apply any styles to ::shadow, i.e. shadow roots.
So if ::shadow is the rightmost selector of a given complex selector,
the complex selector should not match any elements.

BUG=367266
TEST=fast/dom/shadow/shadow-pseudo-element.html

Review URL: https://codereview.chromium.org/266493002

git-svn-id: svn://svn.chromium.org/blink/trunk@173477 bbb929c8-8fbe-4397-9dbb-9b2b20218538