Deprecate DatabaseObserver

- Remove old DatabaseObserver code
- Remove public/web/WebDatabaseObserver.h
- Rename WebPlatformDatabaseObserver (in public/platform) back to
  WebDatabaseObserver (but keeps a typedef of WebPlatformDatabaseObserver
  until chrome side's switch is over)

BUG=none

Review URL: https://codereview.chromium.org/61753016

git-svn-id: svn://svn.chromium.org/blink/trunk@162034 bbb929c8-8fbe-4397-9dbb-9b2b20218538