Web SQL: Remove AbstractDatabaseServer and DatabaseServer.

These classes just add code complexity, and have no benefit. This CL folds them
into DatabaseManager.

BUG=

Review URL: https://codereview.chromium.org/578803002

git-svn-id: svn://svn.chromium.org/blink/trunk@182209 bbb929c8-8fbe-4397-9dbb-9b2b20218538