StyleResolver::appendAuthorStyleSheets should check whether scoping node is really document or not.

Suppose that we are removing two style elements in a shadow tree at the same time:
For example, changing innerHTML from '<style>...</style> ... <style>...</style>' to '',

When one style element is being removed (i.e. in HTMLStyleElement::removedFrom), another one is still in document.
However, containing shadow root of both elements is 0.
So ScopedStyleResolver::scopingNodeFor returns 0 and styles in document will be modified.
This is a problem.

TEST=fast/dom/shadow/remove-styles-in-shadow-crash-2.html

Review URL: https://codereview.chromium.org/33443007

git-svn-id: svn://svn.chromium.org/blink/trunk@160197 bbb929c8-8fbe-4397-9dbb-9b2b20218538