Oilpan: Prepare to move IDBDatabase to Oilpan heap.

IDBDatabase had a non-trivial destructor which accessed other GarbageCollected
objects. We can't do it in Oilpan.

Fortunately we can replace close() with a simple code which doesn't access
GarbageCollected objects because:

 - m_transactions is always empty in the destructor. IDBTransactions stored in
   m_transactions have strong pointers to this IDBDatabase.
 - m_enqueuedEvents is always empty in the destructor. Events stored in
   m_enqueuedEvents have strong references to this IDBDatabase (See
   IDBDatabase:enqueueEvent.)

BUG=366528

Review URL: https://codereview.chromium.org/261103003

git-svn-id: svn://svn.chromium.org/blink/trunk@173278 bbb929c8-8fbe-4397-9dbb-9b2b20218538