Bug 921522: Use already-established flex container content-box main size to determine packing space, instead of checking reflow state. r=mats

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rename : layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-horiz-1.xhtml => layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-horiz-1a.xhtml
rename : layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-horiz-1.xhtml => layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-horiz-1b.xhtml
rename : layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-vert-1.xhtml => layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-vert-1a.xhtml
rename : layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-vert-1.xhtml => layout/reftests/w3c-css/submitted/flexbox/flexbox-justify-content-vert-1b.xhtml