Bug 873172: When checking whether a flex item is generated content, skip past any anonymous boxes around it. r=bz

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rename : layout/reftests/flexbox/flexbox-with-pseudo-elements-1-ref.html => layout/reftests/flexbox/flexbox-with-pseudo-elements-3-ref.html
rename : layout/reftests/flexbox/flexbox-with-pseudo-elements-1.html => layout/reftests/flexbox/flexbox-with-pseudo-elements-3.html