Move WebIDBDatabaseCallbacksImpl out of web/, collapse some impls

Moving the implementation of blink::WebIDBDatabaseCallbacks out of web
and into modules/indexeddb will unblock the removal of other IDB
proxy types in web; WebCore will be able to mint the impl and pass
it straight through to the blink::WebIDBFactory implemented by the
embedder.

R=alecflett@chromium.org,jamesr@chromium.org
BUG=234277

Review URL: https://codereview.chromium.org/70173008

git-svn-id: svn://svn.chromium.org/blink/trunk@162239 bbb929c8-8fbe-4397-9dbb-9b2b20218538