Don't mutate the DOM from inside an Element's constructor

We shouldn't call ensureUserAgentShadowRoot() from inside an element's
constructor since not all constructors in the inheritance chain may
have run, and also since it means going into DOM methods that may want
to ref |this| before it's ever been adopted.

This patch fixes this and also fixes the RefPtr protects inside
notifyNodeInserted.

Review URL: https://codereview.chromium.org/304003002

git-svn-id: svn://svn.chromium.org/blink/trunk@175077 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove tests that shouldn't pass according to the current spec.

Some tests in style-with-shadow-pseudo-element.html and
style-with-deep-combinator.html did check if a style rule
in a shadow tree can be applied to its sibling shadow tree(s),
which is against the current spec.

I will add some test *against* such case, that a style rule
doesn't apply to its sibling shadow tree, along with the fix for
bug 355674 later.

BUG=355674
TEST=pass layout tests.

Review URL: https://codereview.chromium.org/304123002

git-svn-id: svn://svn.chromium.org/blink/trunk@175078 bbb929c8-8fbe-4397-9dbb-9b2b20218538