ui/base/models: Rewrite the TreeNodeModelTest.HasAncestor unittest.
ui/base/models: Rewrite the TreeNodeModelTest.HasAncestor unittest.

- This improves and simplifies the tests covered in HasAncestor.

BUG=None
TEST=ui_unittests --gtest_filter=TreeNodeModelTest.RemoveAllNodes

R=sky@chromium.org

Review URL: http://codereview.chromium.org/7108072

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@88900 0039d316-1c4b-4281-b951-d872f2087c98

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