Oilpan: Prepare to move IDBDatabaseCallbacks to Oilpan heap.

IDBDatabaseCallbacks::m_database should be a WeakMember because
WebIDBDatabaseCallbacksImpl has a strong reference to 
IDBDatabaseCallbacks and WebIDBDatabaseCallbacksImpl can outlive
the associated IDBDatabase. If DBDatabaseCallbacks::m_database was
a strong reference, a lifetime assumption of Indexed DB would be
broken and some tests would fail.


BUG=366528

Review URL: https://codereview.chromium.org/268023002

git-svn-id: svn://svn.chromium.org/blink/trunk@173470 bbb929c8-8fbe-4397-9dbb-9b2b20218538