This does not support key-rotation and will give an error. This is
because WebM does not have a way to indicate a change in key ID using
media segments.
b/22463551
Change-Id: I9b3dac818dc370302a5afc0d25d8a060b64d30cd
* Changed Segmenters to accept StreamInfo rather than MediaStream
to help in testing.
* Changed MemoryFile behavior to mirror local file:
* Read non-existent file is an error.
* Write deletes any existing file.
* Fixed a bug in SingleSegmentSegmenter.
Change-Id: I339e35597ca4661b7a26c6fdbbfa2f9f511c7da0