Document class inherits TreeScope and don't need treeScope() accessor.
Document class inherits TreeScope and don't need treeScope() accessor.

This was brought when the code moved from HTMLDocument to Document in
https://codereview.chromium.org/141733006

TEST=pass all layout tests

Review URL: https://codereview.chromium.org/944123006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190812 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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