(no bug) Update ResolveFlexibleLengths() documentation & drop whitespace on blank line. comment/whitespace-only

(Prior to this commit, the documentation mentioned long-since-removed params aTotalFreeSpace and aItems.)
Bug 992397: Relax extreme testcases in test_flexbox_layout.html to allow an epsilon difference. r=mats
Bug 983427 part 5: Toggle switch to enable the code added in this bug & avoid reflowing flex items in bottom-to-top order. r=mats