I found this while fixing https://webkit.org/b/129655. If you set some breakpoints, navigate away, then click the back button, there will be no breakpoints even listed in the DebuggerSidebarPanel for the page you returned to. It seems to be page cache related. I'm not sure what sequence of events we receive when hitting the page cache- it may be the case that we aren't listening to the right event to restore from the frontend or backend.
*** This bug has been marked as a duplicate of bug 151581 ***
<rdar://problem/23653288>