Bug 841847: Ensure that we reflow a flex container's children, if any of them received a "measuring" reflow. r=dbaron a=lsblakk

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rename : layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-1-iframe.html => layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-2-iframe.html
rename : layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-1-ref.xhtml => layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-2-ref.xhtml
rename : layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-1.xhtml => layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-2.xhtml
rename : layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-1-iframe.html => layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-3-iframe.html
rename : layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-1-ref.xhtml => layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-3-ref.xhtml
rename : layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-1.xhtml => layout/reftests/flexbox/flexbox-dyn-changeFrameWidth-3.xhtml