Add two classes [as yet unused] needed for upcoming IDB Blob support.

BUG=108012
R=cmumford,jsbell,abarth

Review URL: https://codereview.chromium.org/205413004

git-svn-id: svn://svn.chromium.org/blink/trunk@170686 bbb929c8-8fbe-4397-9dbb-9b2b20218538