mkvmuxer: Fix unit'd value in Segment.
mkvmuxer: Fix unit'd value in Segment.

Init ebml_header_size_ to 0. Avoids MSan issues when tests that never
mux samples call Finalize() on Segment.

BUG=crbug.com/1212736

Change-Id: I00b1c0aa8c75eb1a0067eb72ac2bd83021995389

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm