Oilpan: HeapTest: FinalizationObserverWithHashMap should not leave a Persistent object.

Clear the Persistent HeapHashMap at the end of HeapTest.FinalizationObserver.

HeapTest.CheckAndMarkPointer assumes there are no Persistent objects, and
triggers GC without visiting Persistent handles. If HeapTest.CheckAndMarkPointer
is executed after HeapTest.FinalizationObserver, and a normal GC happens,
a Visitor accesses deallocated memory pointed by the Persistent.

BUG=none

Review URL: https://codereview.chromium.org/676603002

git-svn-id: svn://svn.chromium.org/blink/trunk@184238 bbb929c8-8fbe-4397-9dbb-9b2b20218538