mkvwriter: Fix shorten-64-to-32 warning in 32 bit builds.
mkvwriter: Fix shorten-64-to-32 warning in 32 bit builds.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1207

Change-Id: I2dc1ce59ebf8051c9661b919813d5ab7cb115da0

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm