Convert DocumentParser code to Use more reference

Document pointer can not be null to create DocumentParser.
Also HTMLViewSourceParser parameter can not be null. 
Move the parameter to use reference to make caller code safer. This is follow up patch from previous CL# 276733002.

Review URL: https://codereview.chromium.org/277633002

git-svn-id: svn://svn.chromium.org/blink/trunk@174654 bbb929c8-8fbe-4397-9dbb-9b2b20218538