Drop unnecessary NodeWithIndex::m_haveIndex data member

Drop unnecessary NodeWithIndex::m_haveIndex data member. Since
Node::nodeIndex() returns an unsigned integer but NodeWithIndex::m_index is
signed, we can simply set m_index to -1 until it is lazily set.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/311563003

git-svn-id: svn://svn.chromium.org/blink/trunk@175299 bbb929c8-8fbe-4397-9dbb-9b2b20218538