Bug 883814. Make HasLowUndecodedData not use GetBuffered, but instead estimate based on the current playback data rate. r=cpearce,a=leo+

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extra : rebase_source : 6056a78baffd9d4897656fc9d95cc8d0169d6792
Bug 883814. Part 2: Allow buffering to be triggered when we're low on decoded video but not audio. r=cpearce,a=leo+

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extra : rebase_source : 759e8cffc0fe85069430d01c192dd6b7b4495c4c