about:tor and about:tbupdate fail to load in debug build of Tor Browser

When using a debug build based on acat's 33533+5 branch, trying to open about:tor or about:tbupdate leads to an assertion failure and a tab crash: Assertion failure: foundObjectSrc (about: page must contain a CSP denying object-src), at /.../dom/security/nsContentSecurityUtils.cpp:818

We need to add object-src 'none' to the CSP for those pages.