libwebm v1.0.0.24
Parse headers outside of constructor

In several cases, the parser would parse a header
(say, a track header, or the segment info header)
in the constructor for the type.  The parser had
assumed (incorrectly) that the file was well-formed,
but this turned out to be an incorrect assumption.

The parse errors triggered some assertion failures,
but this is not acceptable in a production system.

The parser solved that problem by separating the
construction of the header object from the parsing
of the file.  There is now a separate parse method
to be called after construction of the object,
which returns a status whether the parse was
successful.

This change fixes the bugs from the following
tracker issues:

http://code.google.com/p/webm/issues/detail?id=399
http://code.google.com/p/webm/issues/detail?id=400

Change-Id: Idb09154ae7008429d8613ce3b3e8294f5a12de86
5 files changed