You can try going to CEF Automated Builds and downloading your CEF version’s debug symbol and placing that libcef.dll.pdb next to your libcef.dll. For me it was under UnrealEngine/Engine/Binaries/ThirdParty/CEF3/.
You can also try looking at the logs
My problem ended up being that I was missing .pak files under my Resources\ folder and my Resources\locale folder. So I put a breakpoint at CefInitialize(mainArgs, settings,…) and paid special attention to where my resources and locale paths pointed to. After that, I copied the .pak files from my CEF binary distribution’s Resource folder into the appropriate places and it worked. It’s weird that in the Unreal codebase, they don’t have the Resources folder under the CEF3 binary folder. I wonder where they load them from.