Bug 983427 part 1: Add FlexboxAxisTracker flag to indicate whether we've detected bottom-to-top flow & are internally reversing axes. r=mats
Bug 983427 part 2: If axes are reversed, link up our lists of FlexLines & FlexItems in reverse, too. r=mats
Bug 983427 part 3: If axes are reversed, swap "flex-start" and "flex-end" alignment (& treat "stretch" as a variety of "flex-start"). r=mats
Bug 983427 part 4a: Expand FlexItem baseline accessor to let caller pick the edge as well as the axis. r=mats
Bug 983427 part 4b: If axes are reversed, align baseline-aligned items with respect to to the flex-end edge (which is the "real" flex-start edge). r=mats
Bug 983427 part 4c: If axes are reversed, compute flex container's baseline using the *last* flex line (which in reality is the first line). r=mats