Bug 1109644. Part 1: Create SourceMediaStream::GetEndOfAppendedData to reliably track how much data has been appended to a SourceMediaStream's track. r=jesup

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extra : rebase_source : 385ad9b704d35df7b70a75afe807e363e0929476
Bug 1109644. Part 2: Make MediaEngineWebRTCVideoSource use GetEndOfAppendedData instead of aLastEndTime. r=jesup

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extra : rebase_source : a1b716900bba01b5af1057e03bdc6360e327174b
Bug 1109644. Part 3: Make MediaEngineDefaultVideoSource use GetEndOfAppendedData instead of aLastEndTime. r=jesup

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extra : rebase_source : d2285dad46e5b089b49e5ddce12564d5518c45a3
Bug 1109644. Part 4: Make MediaEngineGonkVideoSource use GetEndOfAppendedData instead of aLastEndTime. r=jesup

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extra : rebase_source : 0899d9e42cd42b2ab4ef1d0d4cf600ccc7f49a3f
Bug 1109644. Part 5: Make MediaEngineTabVideoSource use GetEndOfAppendedData instead of aLastEndTime. r=jesup

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extra : rebase_source : ee34171ed4297afa90fb14c7cf655a8d22a30dca
Bug 1109644. Part 6: Remove aLastEndTime parameter from NotifyPull. r=jesup

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extra : rebase_source : 47c950d1b1d03a8de279f2ac361b8dcd4ab0f801