Followup 418f2606e03b82a5ae0afc853406c7e9f2d1e35f: put <?xml tag back.

These aren't optional in SVG files. Browsers will display them without,
but dropping them breaks MIME type detection, which breaks data URI
embedding: the underline-u icon was being embedded as data:text/plain,...

Bug: 58119
Change-Id: Ia790877fcd536f2714626ccf47beadd09cb4fac9
Merge "Followup 418f2606e03b82a5ae0afc853406c7e9f2d1e35f: put <?xml tag back."