Cleanup obsolete isolated-origin handling code for SandboxOriginDatabse

So that it becomes easier to make more meaningful changes later.
(will come in a separate patch later)

BUG=306030
R=tzik

Review URL: https://codereview.chromium.org/35253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230658 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230658