Bug 246760

I intend to enable DEVELOPER_MODE_FATAL_WARNINGS by default to finally fix bug #155047, since I've been spending way too much time fighting build warnings instead of doing more interesting work, and really need the EWS bots to enforce fatal warnings. But we never want to lose test results to a silly build warning: that would be brutal punishment for somebody introducing a new warning. So for this to work, we need post-commit buildbots (but not EWS) to use build-webkit --no-fatal-warnings. Post-commit bots that don't use build-webkit should use the -DDEVELOPER_MODE_FATAL_WARNINGS=OFF CMake option. 

I'm not sure the best way to do this. We could add additional arguments to every bot in Tools/CISupport/build-webkit-org/config.json, but that would need to be repeated for every single bot. Maybe there is another way? Don't want to change build-webkit because fatal warnings should be default in developer mode. (The name of the option DEVELOPER_MODE_FATAL_WARNINGS doesn't make much sense otherwise.)