Fix a race in DatabaseManager::manager.

DatabaseManager::manager should return a singleton of DatabaseManager. However
it had no mutex. It was possible to create two DatabaseManager.

BUG=378250
NOTRY=true

Review URL: https://codereview.chromium.org/308593004

git-svn-id: svn://svn.chromium.org/blink/trunk@175057 bbb929c8-8fbe-4397-9dbb-9b2b20218538