Reporting use counts by invoking a callback once per occurrence has
a large overhead cost in certain situations, for example when it needs
to be dispatched to a different thread (which is the case for Web Workers).

Parsing large scripts can produce a lot of occurrences (strict/sloppy mode
once per function).

Chromium (the only known user of UseCounters so far) does not actually care
about number of occurrences, but simply whether they happened at least once.
This commit changes behavior to report features at most once, which dramatically
improves performance for impacted use cases, and should not affect the only
known real world usage.

R=littledan@chromium.org
BUG=chromium:614775

Review-Url: https://codereview.chromium.org/2062203002
Cr-Commit-Position: refs/heads/master@{#36979}