IndexedDB: Tag some single-argument constructors as explicit

They're private, but the coding guidelines don't care.

R=dgrogan@chromium.org

Review URL: https://codereview.chromium.org/102703002

git-svn-id: svn://svn.chromium.org/blink/trunk@163134 bbb929c8-8fbe-4397-9dbb-9b2b20218538