Chrome internally uses time accurate to microseconds, which is
implemented per MSE spec (https://www.w3.org/TR/media-source/).
Generate Period@duration with better precision to avoid possible
buffered range gaps in Chrome (possibly other browsers too), which
may lead to other problems like playback stall.
b/74238961
Fixes#368.
Change-Id: I357a0f62b67f75c7ca044bb99ea4e3c8bbb6fecd