Having PlaybackManager start in Buffering was causing us to report
a media element readyState of HAVE_CURRENT_DATA. HAVE_CURRENT_DATA
doesn't make a whole lot of sense for local files, since we should have
all the data immediately when we process the metadata. This is
reflected in the buffered attribute, so let's not limit the ready state
unecessarily.
These steps are the best definition we have for how the ready state
should be set, and it seems to be reasonable to apply to plain file
playback as well.
Since our file demuxers are hardcoded to return the entire duration as
buffered, the ready state immediately progresses to HAVE_CURRENT_DATA.
This will probably change once we can check the demuxers for buffered
data.
Since we know whether we're buffering from the PlaybackManager state,
let's use that to update the ready state. This ensures that when we set
the ready state to HAVE_CURRENT_DATA, we actually have a frame.
Setting the ready state to HAVE_ENOUGH_DATA should probably still be
further conditioned on the buffer size in IncrementallyPopulatedStream,
but this is still an improvement for now.
