[telemetry] Fix type of of media metric.

Missed one call in http://crrev.com/236951 :)
This should make the perf bots green.

BUG=None.
TEST=None.
TBR=shadi

Review URL: https://codereview.chromium.org/84593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236992 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236992