Bug 960873: Part 2: Create AudioCompactor class to minimize allocation slop. r=cpearce

Based on original patch written by :gal.
Bug 960873: Part 3: AudioCompactor gtest unit tests. r=cpearce
Bug 960873: Part 4: Make OMX use AudioCompactor. r=cpearce
Bug 960873: Part 5: Make gstreamer use AudioCompactor. r=cpearce
Bug 960873: Part 6: Make DirectShow use AudioCompactor. r=cpearce
Bug 960873: Part 7: Reduce apple mp3 MAX_AUDIO_FRAMES to minimize alloc slop. r=cpearce