Accurately measure current persistent count while running termination GC.

Follow up on r392263 and have it really behave as wanted -- when measuring
|currentCount|, the live persistent count, while cleaning out the worker's
heap in ThreadState::runTerminationGC(), we need to initially sample it
_after_ having released thread-local static persistents. Otherwise the count
would be overestimated first time around, which could lead to prematurely
reaching a fixed point. And at a fixed point that would falsely indicate
that persistents were leaking.

R=haraken
BUG=
NOTRY=true

Review-Url: https://codereview.chromium.org/1963453002
Cr-Commit-Position: refs/heads/master@{#392272}
1 file changed