Prepare for deprecating DatabaseObserver.{h,cpp}

For fixing layering violation (declared in WebCore, implemented in Source/web)

1. Add DatabaseClient for canEstablishDatabase()
2. Add WebPlatformDatabaseObserver in platform layer for remaining methods

BUG=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/64353002

git-svn-id: svn://svn.chromium.org/blink/trunk@161587 bbb929c8-8fbe-4397-9dbb-9b2b20218538