Rename TreeScope::documentScope() to document()

The only problem this causes is an ambiguity in ShadowRoot and Document,
which now inherit document() methods from two base classes.

For ShadowRoot, I solved this by adding a 'using' declaration. For Document,
I simply replaced all calls to Document::document() with simpler code (there's
no need to call document() on an object whose type is statically Document,
since it always returns |this|).

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/126203002

git-svn-id: svn://svn.chromium.org/blink/trunk@164609 bbb929c8-8fbe-4397-9dbb-9b2b20218538