DocumentStyleSheetCollection::removeStyleSheetCandidateNode should use scopingNode's treeScope.

Since a given node might have been already removed from DOM tree, node->treeScope() might not be available.
Instead, scopingNode->treeScope() should be used.

BUG=270272

TEST=fast/dom/shadow/remove-styles-in-shadow-crash.html covers.
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/22440005

git-svn-id: svn://svn.chromium.org/blink/trunk@155830 bbb929c8-8fbe-4397-9dbb-9b2b20218538