Gracefully deal with clearing content settings for unregistered extensions.

This CL is only for merging to the M20 branch; on trunk I'm going to revert it and add additional information to the crash dumps to find out why this is happening.

BUG=128652
TEST=no crash


Review URL: https://chromiumcodereview.appspot.com/10383303

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@141415 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed