Bug 783470 part 1: Split most of flex reflow into a helper-function, so that we can easily restart it by reinvoking that helper.
Bug 783470 part 2: Create an array with information about each visibility:collapse "strut" that we need to build.
Bug 783470 part 3: Use the strut metadata we've saved to actually build special "strut" FlexItems on our second pass through reflow.
Add reftests for collapsed flexbox items.
Add assertions for absolutely positioned children.
Merge branch 'multiline_flexbox-work'