Move MSE and EME trace events to "media" category.

TBR=dmichael@chromium.org
BUG=none
TEST=none

Review URL: https://codereview.chromium.org/68213008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234473 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234473