Bug 1054161. Don't build display items for text with alpha=0. r=dbaron

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extra : rebase_source : 869a05a652a3bc6e8e122d34fb166ed9fbbdaa5c
Bug 1048579. Always sniff for media loads. r=bzbarsky

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extra : rebase_source : 067e99eaad781dec78a772405870b02c195a42e2