This revision adds an operator<< that writes a text representation of an
AccAttributes object to an ostream object. This makes it possible to call
ToString on an AccAttributes object, which is useful for printing AccAttributes
objects embedded in AccAttributes objects, and relevant to how Gecko stores
ARIA and text attributes. This revision rewrites DebugPrint in terms of ToString
and uses ToString in StringFromNameAndValue to print AccAttributes in AccAttributes.
Differential Revision: https://phabricator.services.mozilla.com/D224558