telemetry: re-enable smoothness.key_mobile_sites

This benchmark should work when crrev.com/68763011 has landed.

R=tonyg@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/79883003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236550 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236550