Remove manual ref()-deref() in DatabaseContext.

Remove manual ref()-deref() in DatabaseContext.

DatabaseManager holds DatabaseContext references instead.

- We need to move DatabaseManager::registerDatabaseContext call from the
  constructor to the factory because registerDatabaseContext now increments the
  reference counter of a DatabaseContext and we can't update the reference
  counter in the constructor.

- We move DatabaseManager::unregisterDatabaseContext call from stopDatabases to
  contextDestroyed in order to match to deref() in the old code.

- We don't need m_isRegistered any longer because contextDestroyed is called
  just once. stopDatabases can be called twice.

- We need to move ~DatabaseManager implementation to DatabaseManager.cpp because
  we add RefPtr<DatabaseContext> in a member of DatabaseManager and
  ~DatabaseManager needs a complete definition of DatabaseContext.

BUG=347902
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170027

[tkent, 2014-03-26 05:42:16]

Please review this.

[haraken, 2014-03-26 05:54:13]

LGTM. This CL makes the ownership of DatabaseContext much clearer.

[tkent, 2014-03-26 05:55:15]

The CQ bit was checked by tkent@chromium.org

[commit-bot: I haz the power, 2014-03-26 05:55:21]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tkent@chromium.org/212133002/1

[commit-bot: I haz the power, 2014-03-26 06:42:39]

Change committed as 170027