Add blob-writing functionality [as yet un-called] to IDB's backend.

      BUG=108012
R=cmumford@chromium.org, jsbell@chromium.org

Review URL: https://codereview.chromium.org/240003011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266916 0039d316-1c4b-4281-b951-d872f2087c98