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Before the change, the database would contain leftover "namespace-namespaceid-"
keys with no areas. This change ensures that they are not left over, and that
the existing leftover keys are scavenged. (This will delete the leftover data
for users who have it.)

BUG=175232
R=michaeln


Review URL: https://chromiumcodereview.appspot.com/12224092

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@182259 0039d316-1c4b-4281-b951-d872f2087c98