Oilpan: Only add trace events for actual sweeps of the heap.

If sweep is not requested performPendingSweep is a noop and that will
distort the picture that we get from tracing. We only want to trace
when we are actually sweeping the heap so that we can get data on
the average sweep times.

R=erik.corry@gmail.com, haraken@chromium.org

Review URL: https://codereview.chromium.org/288973003

git-svn-id: svn://svn.chromium.org/blink/trunk@174272 bbb929c8-8fbe-4397-9dbb-9b2b20218538