Web SQL: Remove unnecessary abstraction.

This CL merges
 - AbstractSQLStatement into SQLStatement
 - AbstractSQLStatementBackend into SQLStatementBackend
 - AbstractSQLTransaction into SQLTransaction
 - AbstractSQLTransactionBackend into SQLTransactionBackend
 - DatabaseBase into Database.

'Sync' versions of their subclasses were removed. These abstract classes are
unnecessary.

BUG=397429
TEST=none; No behavior changes.

Review URL: https://codereview.chromium.org/561143002

git-svn-id: svn://svn.chromium.org/blink/trunk@181809 bbb929c8-8fbe-4397-9dbb-9b2b20218538