Use convertToCSSPropertyID helper in UseCounter.
Use convertToCSSPropertyID helper in UseCounter.

So use convertToCSSPropertyID() helper to hide a static_cast. Also fix
incorrect comment: make it refer to maximumCSSSampleId(). ASSERT that
CSSPropertyInvalid and CSSPropertyVariable should not be present in use
counts, matching the behavior of UseCounter::updateMeasurements().

BUG=None

Review URL: https://chromiumcodereview.appspot.com/14696008

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@149557 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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