Add a comment about UseCounter availability in M35 branch.

We have similar comments for M33 and M34.

NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/220513002

git-svn-id: svn://svn.chromium.org/blink/trunk@170514 bbb929c8-8fbe-4397-9dbb-9b2b20218538