Bug 1061046. Part 1.5: Expose MediaStream::GraphRate(). r=karlt
This is guaranteed to work even if the stream has been disconnected from the graph,
so it's preferred to Graph()->GraphRate().

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extra : rebase_source : e12fe80b3d95eea5f63e460bf557518a5bdcd42c
bug 1083038 be more careful about conversion of seconds to floating point ticks r=padenot

Audio TrackTicks now match StreamTime so it is no longer necessary to use
double seconds to convert from StreamTime to TrackTicks.
Instead leave the fractional time in units of ticks/StreamTime, so that the
fractional offset is calculated from the quantity that is rounded.

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extra : rebase_source : 06bbb3273b7fb9f222e259bfb19af6a2db8224cf