Remove minidump analysis aides from URLRequestThrottlerManager. New

Remove minidump analysis aides from URLRequestThrottlerManager. New
minidumps are not giving us any extra information. We've confirmed
that this is a memory overwrite coming from somewhere else, and to
catch it we will most likely need to reproduce it under a memory
debugging tool.

I've left in the guard against null values as it seems to avoid the
crash and since we aren't getting more information from additional
minidumps for this particular crash I think it's best to lessen the
impact on our users.

BUG=71721
TEST=net_unittests