Add comments to UseCounter.h
Add comments to UseCounter.h

- A comment about M36 branch
- Comments about counting in workers.

NOTRY=true
BUG=376039

Review URL: https://codereview.chromium.org/292973004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@174501 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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