||This structure is used to store information logged by various gecko
components for later examination by test code.
It contains a bucket for every paint (initiated on the client side),
and every repaint request (initiated on the compositor side by
AsyncPanZoomController::RequestContentRepait), which are identified by
sequence numbers, and within that, a set of arbitrary string key/value
pairs for every scrollable frame, identified by a scroll id.
There are two instances of this data structure for every content thread:
one on the client side and one of the compositor side.
It also contains a list of hit-test results for MozMouseHittest events
dispatched during testing. This list is only populated on the compositor
instance of this class.