Fix about:tor assertion failure in esr68 linux debug builds

I found this assertion failure when testing legacy/trac#30429 (moved) in linux, debug build. It happens when loading about:tor:

Assertion failure: foundDefaultSrc (about: page must contain a CSP including default-src), at /home/user/tor/tor-browser/dom/base/Document.cpp:5179

We should investigate this, but not sure if it's tbb-9.0-must-nightly.