Web SQL: Move almost all of Database functions to DatabaseBackend.

This is the first CL to merge DatabaseBackend and Database. The next CL will
remove the existing Database.{cpp,h}, and rename DatabaseBackend.{cpp,h} to
Database.{cpp,h}.

TEST=none; No behavior changes
BUG=

Review URL: https://codereview.chromium.org/583933002

git-svn-id: svn://svn.chromium.org/blink/trunk@182382 bbb929c8-8fbe-4397-9dbb-9b2b20218538