Simplify Node::isDescendantOf TreeScope check
Simplify Node::isDescendantOf TreeScope check

Node::isDescendantOf has a round about way of checking that two nodes are
in the same TreeScope so when doing document.contains(node) where node
is in a ShadowRoot it returns false. This can be greatly simplified and
we can make a.contains(b) O(1) when a and b are in different ShadowRoots
or Documents.

Review URL: https://chromiumcodereview.appspot.com/17582011

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@152956 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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