DiscardRemaning was needed to prevent debug-time assertion that the buffer was
read completely or explicitly discarded.
However this required extra work in cases where buffer didn't need to be read
to the end.
And also it could cause crashes (in debug versions) if a buffer was not fully
read, be it because the parser was incorrect or because the media file itself
was wrong (though possibly still readable despite that).
Finding parser issues is still possible by manually instrumenting ByteReader
during development.
And reading media file with small recoverable errors is a bonus.
MozReview-Commit-ID: 2RUYzaYAeRW
--HG--
extra : rebase_source : 26c41758b1b2c87542bf4e41d08e361198ca5b13
df2f457e83