Oilpan: blink_heap_unittests: asan check failed. HeapObjectHeader::finalize zap a wrong memory for object without VTable

1. In the HeapObjectHeader::finalize, we zap the primary vTable entry, but we have not checked if the object has vTable.

R=ager@chromium.org, oilpan-reviews@chromium.org
BUG=411204

Review URL: https://codereview.chromium.org/547903002

git-svn-id: svn://svn.chromium.org/blink/trunk@181596 bbb929c8-8fbe-4397-9dbb-9b2b20218538