py3: for statprof's Chrome output, write json to string, then encode to bytes `json.dump(obj, fp)` requires `fp.write()` to accept str output, and since the file pointer we have there only accepts bytes, we need to change to json.dumps() and then encode as utf-8. We have already done the same thing for the json (non-Chrome) format in 4b7eb862692e (py3: encode json output to bytes and use write(), 2018-10-12). Differential Revision: https://phab.mercurial-scm.org/D6781