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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