IndexedDB: Test empty-string as database/object store/index name

Follow on to crrev.com/189413007 that removes a bogus
DCHECK about empty strings on the Chromium side.

BUG=350133
TBR=dgrogan
NOTRY=true

Review URL: https://codereview.chromium.org/191593002

git-svn-id: svn://svn.chromium.org/blink/trunk@168849 bbb929c8-8fbe-4397-9dbb-9b2b20218538