Use IDL argument default values in Document.idl

The 'whatToShow' and 'filter' arguments to the createNodeIterator() and
createTreeWalker() methods have default values in the specification.
Add those default values in Document.idl, and get rid of the extra
overloads of Document::createNodeIterator() and ::createTreeWalker().

BUG=258153

Review URL: https://codereview.chromium.org/342403007

git-svn-id: svn://svn.chromium.org/blink/trunk@176938 bbb929c8-8fbe-4397-9dbb-9b2b20218538