Unit tests which just load data: URLs create a loader without a resource dispatcher. When ClusterFuzz fuzzes these to HTTP URLs it hits nullptr derefs. I set out to fixed a random one in  Issue 859354  but we could systematically fix this by using a stub resource dispatcher in unit tests.

This nice idea came from ricea in https://chromium-review.googlesource.com/c/chromium/src/+/1220870#message-e52690da1c6d0fff9a4cf37f6d5bfd27e36e8ae8