mkvmuxer: Set is_key to true for metadata blocks.
mkvmuxer: Set is_key to true for metadata blocks.

Per matroska semantics, all metadata blocks should be considered
key frames. Set it to true in AddMetadata functions.

Change-Id: Ibc26845dc6cc72ccbf90ae8d4dcc27c948e8a375

webmproject/libwebm,webmproject/libwebm,felipebetancur/libwebm,felipebetancur/libwebm,zofuthan/libwebm,zofuthan/libwebm,webmproject/libwebm