Add a comment about counter availability in M37 branch.
Add a comment about counter availability in M37 branch.

NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/357483006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@176903 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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