Use Cues to seek Matroska files

On the first seek, load the Cues element. Parse it incrementally until
the desired seek point can be located.

This allows files to begin playing immediately. However, the Browser
still seeks to 0 before playing embedded YouTube files. Because YouTube
stores the cues at the end of the file, this causes it to seek, load the
cues, then begin playing. It is still better than the previous behavior
which blocked until the entire file was loaded.

BUG=5921311

Change-Id: Iad2abc64ded3b4e2c2d2c478a969f68450754282