Fix the memory leak in test case RawPtrInHash of blink_heap_unittests

1. Reason: running blink_heap_unittests with valgrind or Lsan will report this memory leak in report, so fix it.
2. Modification: after the check operation delete the allocated memory pointed by raw pointer in the set element.

R=haraken@chromium.org, oilpan-reviews@chromium.org
BUG=397452

Review URL: https://codereview.chromium.org/423563003

git-svn-id: svn://svn.chromium.org/blink/trunk@179468 bbb929c8-8fbe-4397-9dbb-9b2b20218538