bug 1098126 re-enable mediasource-redundant-seek and mediasource-duration with new results

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extra : rebase_source : 441f0ee88fc96c3f81628ab2f65b2d082b3bb4df
bug 975782 bring media resource loads out of background while they delay the load event r=cpearce

This will happen after a stalled load doesn't delay the load event but such a
load then delays the load event again when it receives progress.

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extra : rebase_source : a04dd7416f86306cfc62aabba20fb30415572d98
bug 975782 stop delaying the load event when media fetch has stalled r=cpearce,f=kinetik

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extra : rebase_source : c26dcd4221ba051ffff417631ae7ad9b7b65995d