Oilpan: Replace TRACE_EVENT in postSweep() with TRACE_COUNTER
Oilpan: Replace TRACE_EVENT in postSweep() with TRACE_COUNTER

The TRACE_EVENT in postSweep is used just to record a collection rate
of Oilpan's GC. It is better to use TRACE_COUNTER than TRACE_EVENT.

BUG=475801
NOTRY=true

Review URL: https://codereview.chromium.org/1287033010

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200522 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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