Disable tracing crash uploader default endpoints

[pilgrim-brave]

about:tracing has options to upload trace crash logs to an endpoint which, by default, is a Google-owned server. You can override the endpoint in 2 different ways:

1. In brave://flags#trace-upload-url you can select a "tag" which changes the endpoint to a URL specified in chrome/browser/about_flags.cc (all options are Google-owned)
2. On the command line, you can specify an arbitrary endpoint with the --trace-upload-url flag

We should keep the command line feature available for power users, but we should disable all the default endpoints or point them to Brave-controlled servers.