Add two classes [as yet unused] needed for upcoming IDB Blob support.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@170686 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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