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: svn://svn.chromium.org/blink/trunk@174501 bbb929c8-8fbe-4397-9dbb-9b2b20218538