Debugging Chrome JS In Pernosco

1. Download our Chromium build (it only has a couple of trivial patches, we can provide the patches if you want to make your own build; at some point those patches will hopefully go upstream)
2. Untar Chrome build into a clean directory <chrome-dir>.
3. Download and install latest rr master.
4. Record Chrome using a command line based on the following:

rr <chrome-dir>/chrome --js-flags="--prof --prof-sampling-interval 1000000 --jitless --expose-gc --no-compilation-cache" --enable-logging=stderr --v8-cache-options=none ...

Chrome will be slow. Sometimes, very slow. If things don't work at all, let us know. Sometimes terminating Chrome with ctrl-C or by other means may not work. The most reliable way right now is to send SIGTERM to the rr process from another terminal.