Auto merge of #16458 - stshine:sequential-fallback, r=pcwalton,emilio

layout: Force reflow in the sequential fallback of block format context

When reflowing a block format context during the inorder traversal,
propagate restyle damage manually to its children since they were
already reflowed. Also, test the border box to see if it can fit into
floats according to CSS 2.1 § 9.5.

Improves reddit and yahoo.

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