Oilpan: Prepare to move IDBTransaction to Oilpan heap.

This CL changes IDBTransactionTest a lot because it checks refCount.
We need to update it so that it checks an object liveness after GC.

Note:
- IDBObjectStore::transaction and IDBRequest::transaction are simple
  accessors. We don't need to use PassRefPtr.

BUG=366528

Review URL: https://codereview.chromium.org/250923011

git-svn-id: svn://svn.chromium.org/blink/trunk@172957 bbb929c8-8fbe-4397-9dbb-9b2b20218538