Oilpan: blink_heap_unittests: Test case have memory leak checked by Lsan

1. For case TraceIfNeeded, modify TraceIfNeededTester to inherit from GarbageCollectedFinalized and at the end of test case TraceIfNeeded trigger a GC.
   After GC, the destructor of TraceIfNeededTester will dereference the RefPtr<OffHeapInt> instance, stop the leak.
2. For case MapWithCustomWeaknessHandling and MapWithCustomWeaknessHandling2, need clear the Persistent handle and trigger a GC at the end of test case.

R=ager@chromium.org, oilpan-reviews@chromium.org
BUG=401018

Review URL: https://codereview.chromium.org/452723002

git-svn-id: svn://svn.chromium.org/blink/trunk@179938 bbb929c8-8fbe-4397-9dbb-9b2b20218538