mkvmuxer can write matroska v4 elements in all modes, but does
not always confirm that the doc type is high enough for all
elements written. The main culprits are Colour and its
children. Avoid problems by using 4 as the default version.
This avoids problems in all cases but those where users who
know what they are doing have written the EBML header manually.
Update the test files because the new default causes the file
compare abuse in the muxer tests to fail.
Update expected doc type version in the parsing tests.
BUG=webm:1201
Change-Id: I4c69000af4c1d5efe66315c17f3953344ef04993
