Reland of PyLint fixes for tools-webrtc and webrtc/tools (patchset #1 id:1 of https://codereview.webrtc.org/2737233003/ )

Reason for revert:
Fixing errors for reland. I have tested that this does not make Chromium video quality tests fail.

Original issue's description:
> Revert of PyLint fixes for tools-webrtc and webrtc/tools (patchset #3 id:40001 of https://codereview.webrtc.org/2736233003/ )
>
> Reason for revert:
> Fails video quality tests in Chrome:  http://build.chromium.org/p/chromium.webrtc.fyi/builders/Win10%20Tester/builds/6568
> I should have looked more closer at those :(
>
> Original issue's description:
> > PyLint fixes for tools-webrtc and webrtc/tools
> >
> > Fix a lot of errors before bringing in the new config in
> > https://codereview.webrtc.org/2737963003/
> >
> > BUG=webrtc:7303
> > NOTRY=True
> >
> > Review-Url: https://codereview.webrtc.org/2736233003
> > Cr-Commit-Position: refs/heads/master@{#17137}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/f5318e1f391859c82aaa47b297429e50f41f6b3c
>
> TBR=oprypin@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7303
>
> Review-Url: https://codereview.webrtc.org/2737233003
> Cr-Commit-Position: refs/heads/master@{#17142}
> Committed: https://chromium.googlesource.com/external/webrtc/+/94f4d9effc3fa1ee6a6a48a7bbdef6684defd1dc

NOTRY=true
BUG=webrtc:7312

Review-Url: https://codereview.webrtc.org/2741733003
Cr-Commit-Position: refs/heads/master@{#17541}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 9e6a765..fb79492 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -126,7 +126,7 @@
         non_existing_paths)]
   return []
 
-api_change_msg = """
+API_CHANGE_MSG = """
 You seem to be changing native API header files. Please make sure that you:
   1. Make compatible changes that don't break existing clients. Usually
      this is done by keeping the existing method signatures unchanged.
@@ -155,7 +155,7 @@
           files.append(f)
 
   if files:
-    return [output_api.PresubmitNotifyResult(api_change_msg, files)]
+    return [output_api.PresubmitNotifyResult(API_CHANGE_MSG, files)]
   return []
 
 
@@ -200,7 +200,7 @@
   return []
 
 
-def _CheckNoFRIEND_TEST(input_api, output_api):
+def _CheckNoFRIEND_TEST(input_api, output_api):  # pylint: disable=invalid-name
   """Make sure that gtest's FRIEND_TEST() macro is not used, the
   FRIEND_TEST_ALL_PREFIXES() macro from testsupport/gtest_prod_util.h should be
   used instead since that allows for FLAKY_, FAILS_ and DISABLED_ prefixes."""
@@ -466,13 +466,13 @@
 
 
 def _RunPythonTests(input_api, output_api):
-  def join(*args):
+  def Join(*args):
     return input_api.os_path.join(input_api.PresubmitLocalPath(), *args)
 
   test_directories = [
-      join('webrtc', 'tools', 'py_event_log_analyzer')
+      Join('webrtc', 'tools', 'py_event_log_analyzer')
   ] + [
-      root for root, _, files in os.walk(join('tools-webrtc'))
+      root for root, _, files in os.walk(Join('tools-webrtc'))
       if any(f.endswith('_test.py') for f in files)
   ]