Implement ::shadow pseudo element and replace /shadow/ with ::shadow.

If an element has at least one shadow tree, the ::shadow pseudo-element matches the shadow roots themselves.

c.f. Shadow DOM styling spec:
http://drafts.csswg.org/css-scoping/#shadow-pseudoelement

The new spec doesn't support /shadow/ combinator. Instead, it supports ::shadow.

BUG=355954
TEST=fast/dom/shadow/content-pseudo-element-with-hat.html,fast/dom/shadow/querySelector-for-multiple-shadowroots.html,fast/dom/shadow/querySelector-for-useragent-shadowroot.html,fast/dom/shadow/querySelector-with-shadow-all-and-shadow-deep.html,fast/dom/shadow/style-with-hat.html

Review URL: https://codereview.chromium.org/210713002

git-svn-id: svn://svn.chromium.org/blink/trunk@170131 bbb929c8-8fbe-4397-9dbb-9b2b20218538