Previous implementation was a DOMString. This change also validates values
when constructing IDBVersionChangeEvent instances only allowing the
correct enumeration values.

BUG=358067

Review URL: https://codereview.chromium.org/386883008

git-svn-id: svn://svn.chromium.org/blink/trunk@178517 bbb929c8-8fbe-4397-9dbb-9b2b20218538