Going further from the previous changes, all libfuzzer_main really does
is call the init function, and then proceed to call the fuzzer driver
with the testing function.
So instead of calling that function for it to do all that, the
LibFuzzerRunner can just call the init function itself, and then
call the fuzzer driver with the testing function.
The LibFuzzerRunner code lives in libxul. It's unnecessary complications
to have it call back a function in the firefox executable just so that
it calls another function that is in libxul. Passing the init and
testing functions to the libfuzzer_main function allows to just bypass
that roundtrip, simplifying the setup.
