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.
The function given to XRE_LibFuzzerSetMain is called from somewhere that
does have access to argc/argv already, so we can avoid passing them
to XRE_LibFuzzerSetMain.
This actually might fix subtle issues with argc/argv not really matching
reality when calling the LibFuzzerMain function in the current code:
some arguments are handled before the call, and both argc and argv are
modified from within XRE_main, but the values stored for the
LibFuzzerMain call still are the original ones.
Argv being a pointer, and it not being reallocated, the value stored for
the LibFuzzerMain call points to the changed one, but argc, being an
integer, is not modified accordingly.
In fact, it's actually worse, because while the Gecko code doesn't
reallocate argv, gtk_main might. So if some GTK flag is passed on the
command line, there's also a possibility that the LibFuzzerMain function
will do a use-after-free.
So all in all, it's just better to use the set of modified argc/argv
from XRE_main instead of storing them from main().
