Adding PRESUBMIT check on google::protobuf

The goal is to avoid direct usage of google::protobuf.

It should only be used with a 'using' directive in the header file:
//webrtc/base/protobuf_utils.h.

BUG=webrtc:7340
NOTRY=True

Review-Url: https://codereview.webrtc.org/2753823003
Cr-Commit-Position: refs/heads/master@{#17467}

PRESUBMIT.py

@@ -487,6 +487,26 @@ def _RunPythonTests(input_api, output_api):
   return input_api.RunTests(tests, parallel=True)
 
 
+def _CheckUsageOfGoogleProtobufNamespace(input_api, output_api):
+  """Checks that the namespace google::protobuf has not been used."""
+  files = []
+  pattern = input_api.re.compile(r'google::protobuf')
+  proto_utils_path = os.path.join('webrtc', 'base', 'protobuf_utils.h')
+  for f in input_api.AffectedSourceFiles(input_api.FilterSourceFile):
+    if f.LocalPath() in [proto_utils_path, 'PRESUBMIT.py']:
+      continue
+    contents = input_api.ReadFile(f)
+    if pattern.search(contents):
+      files.append(f)
+
+  if files:
+    return [output_api.PresubmitError(
+        'Please avoid to use namespace `google::protobuf` directly.\n'
+        'Add a using directive in `%s` and include that header instead.'
+        % proto_utils_path, files)]
+  return []
+
+
 def _CommonChecks(input_api, output_api):
   """Checks common to both upload and commit."""
   results = []
@@ -555,6 +575,7 @@ def _CommonChecks(input_api, output_api):
   results.extend(_CheckUnwantedDependencies(input_api, output_api))
   results.extend(_CheckJSONParseErrors(input_api, output_api))
   results.extend(_RunPythonTests(input_api, output_api))
+  results.extend(_CheckUsageOfGoogleProtobufNamespace(input_api, output_api))
   return results