Progress HTMLDocumentParser state on non-interrupted cases

Before this patch, HTMLDocumentParser never progressed to StoppedState
but stuck in StoppingState when parsing finished without being interrupted.
This patch adds a call to |DocumentParser::stopParsing()| to ensure its
|m_state| progress to StoppedState.

BUG=409461

Review URL: https://codereview.chromium.org/532343002

git-svn-id: svn://svn.chromium.org/blink/trunk@181346 bbb929c8-8fbe-4397-9dbb-9b2b20218538