"is_official_build" worked. Didn't use it earlier as it was inside the section for official chrome branded builds and mentioned it required src-internal. But apparently that's only for Chrome branding and the is_official_build flag can be used in public source and it also controls a bunch of optimizations.

For completeness, I had to also use "chrome_pgo_phase = 0" and set "sudo sysctl -w vm.max_map_count=262144" on linux as per the documentation.