Web SQL: Merge DatabaseBackendBase into DatabaseBackend.

We remove DatabaseBackendSync. This abstraction is unnecessary.

BUG=397429

Review URL: https://codereview.chromium.org/576923003

git-svn-id: svn://svn.chromium.org/blink/trunk@182139 bbb929c8-8fbe-4397-9dbb-9b2b20218538