CRM-13918 CRM_Report_Form - variable scope, is not defined in 4.4.3, I added it...

CRM-13918 CRM_Report_Form - variable scope,  is not defined in 4.4.3, I added it for 4.4.4 but as public. I think this was incorrect so changing to protected
I looked for instances in Core & all referred to ->_output. It seems that defining it will cause probs for anyone who has already defined a difference scope in their extension nsio
(either way) so should just do it once right

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* CRM-13918: Add Report.getrows api
  http://issues.civicrm.org/jira/browse/CRM-13918