Reland: Add in missing protobuf code.

Turns out the import of histogram_pb2 failed not on the stub itself
(which I thought for a long, long time), but because of the protobuf
support code it includes in turn. This is a drawback of catching
the ImportError in histogram_proto.py.

This has a decent chance of fixing the problem.

Tbr: mbonadei@webrtc.org
Bug: chromium:1029452
Change-Id: If7ae2439b01ad1b3129d8cc8b158385101082e6f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171867
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30917}

audio/test/low_bandwidth_audio_test.py

@@ -234,8 +234,10 @@ def _ConfigurePythonPath(args):
   sys.path.insert(0, histogram_proto_path)
 
   # Fail early in case the proto hasn't been built.
-  from tracing.proto import histogram_proto
-  if not histogram_proto.HAS_PROTO:
+  try:
+    import histogram_pb2
+  except ImportError as e:
+    logging.exception(e)
     raise ImportError('Could not find histogram_pb2. You need to build the '
                       'low_bandwidth_audio_perf_test target before invoking '
                       'this script. Expected to find '
@@ -245,6 +247,7 @@ def _ConfigurePythonPath(args):
 def main():
   # pylint: disable=W0101
   logging.basicConfig(level=logging.INFO)
+  logging.info('Invoked with %s', str(sys.argv))
 
   args = _ParseArgs()