remove DocumentParser::startParsing()

This CL removes |DocumentParser::startParsing()|.
This was only used from XMLDocumentParser, but it was a no-op.
The method initializes |m_state| to |ParsingState|, however the state should
already be in |ParsingState| from c-tor initializer.

BUG=404107

Review URL: https://codereview.chromium.org/471423002

git-svn-id: svn://svn.chromium.org/blink/trunk@180425 bbb929c8-8fbe-4397-9dbb-9b2b20218538