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

mercurial/statprof.py

@@ -875,7 +875,10 b' def write_to_chrome(data, fp, minthresho'
               if idx not in blacklist]
     frames = collections.OrderedDict((str(k), v)
                                      for (k,v) in enumerate(id2stack))
-    json.dump(dict(traceEvents=events, stackFrames=frames), fp, indent=1)
+    data = json.dumps(dict(traceEvents=events, stackFrames=frames), indent=1)
+    if not isinstance(data, bytes):
+        data = data.encode('utf-8')
+    fp.write(data)
     fp.write('\n')
 
 def printusage():