Bug 783470 part 1: Split most of flex reflow into a helper-function, so that we can easily restart it by reinvoking that helper. r=mats
Bug 783470 part 2: Create an array with information about each visibility:collapse "strut" that we need to build. r=mats
Bug 783470 part 3: Use the strut metadata we've saved to actually build special "strut" FlexItems on our second pass through reflow. r=mats
Bug 783470 part 4: reftests for visibility:collapse on flex items. r=mats
Bug 783470 part 5: Add assertion to ensure we don't create FlexItems for abspos children. rs=mats