Oilpan: Prepare to move IndexedDBClient to Oilpan heap.

- IndexedDBClient and IndexDBClientImpl has no members. It can be
  GarbageCollected<>, and we need to remove the destructor if ENABLE(OILPAN).

- This CL removes WorkerGlobalScopeIndexDatabase::m_client. It was created with
  an IDBFactory, and never used in WorkerGlobalScopeIndexDatabase. So we just
  pass the ownership of the created IndexedDBClient to IDBFactory::create.

BUG=366528

Review URL: https://codereview.chromium.org/251683002

git-svn-id: svn://svn.chromium.org/blink/trunk@172573 bbb929c8-8fbe-4397-9dbb-9b2b20218538