Set X-Content-Duration header on ogg transcodes.

Needed by Firefox. Also wanted by ogv.js. Previously we only
set the header on the source file, not the transcoded files.

This change requires core change Ic053be63b3e

Bug: 62928
Change-Id: I37dc0a6d5d7d0407d883d236e862fd8a33a6d63f
Merge "Set X-Content-Duration header on ogg transcodes."