Web SQL: Remove Database class, and DatabaseBackend should work as Database implementation.

This is a step to merge DatabaseBackend and Database. We need to remove
Database.{cpp,h} before renaming DatabaseBackend.{cpp,h} to Database.{cpp,h} to
keep revision hisotry correctly.

The next CL will rename DatabaseBackend to Database. This CL adds |#define
Database DatabaseBackend| to minimize the size of CLs.

TEST=none; No behavior changes.
BUG=

Review URL: https://codereview.chromium.org/589023002

git-svn-id: svn://svn.chromium.org/blink/trunk@182402 bbb929c8-8fbe-4397-9dbb-9b2b20218538