GN: Add PRESUBMIT.py check for GN changes + default bots.

Add the GN trybots to the default set and also set them
to be the only bots to run if a CL contains only BUILD.gn
changes.

Update Python exclusions in general and fix a few of the lint
warnings.
The ones in python_charts needs to be disabled since those variables
are actually used when passed via vars() to the template.

BUG=None
TEST=git cl presubmit with the following cases:
A CL with two .gyp changes.
A CL with no changes in .gyp* files.

R=niklas.enbom@webrtc.org, phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18719004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6834 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/tools/python_charts/webrtc/main.py b/tools/python_charts/webrtc/main.py
index c2e2b82..90c9ba0 100644
--- a/tools/python_charts/webrtc/main.py
+++ b/tools/python_charts/webrtc/main.py
@@ -101,18 +101,21 @@
   # Loading it into gviz_api.DataTable objects and create JSON strings.
   description, data = helper.CreateConfigurationTable(test_configurations)
   configurations = gviz_api.DataTable(description, data)
-  json_configurations = configurations.ToJSon()
+  json_configurations = configurations.ToJSon()  # pylint: disable=W0612
 
   description, data = helper.CreateData('ssim')
   ssim = gviz_api.DataTable(description, data)
+  # pylint: disable=W0612
   json_ssim_data = ssim.ToJSon(helper.GetOrdering(description))
 
   description, data = helper.CreateData('psnr')
   psnr = gviz_api.DataTable(description, data)
+  # pylint: disable=W0612
   json_psnr_data = psnr.ToJSon(helper.GetOrdering(description))
 
   description, data = helper.CreateData('packets_dropped')
   packet_loss = gviz_api.DataTable(description, data)
+  # pylint: disable=W0612
   json_packet_loss_data = packet_loss.ToJSon(helper.GetOrdering(description))
 
   description, data = helper.CreateData('bit_rate')
@@ -129,6 +132,7 @@
   for row in data:
     row['desired_bit_rate'] = desired_bit_rate
   bit_rate = gviz_api.DataTable(description, data)
+  # pylint: disable=W0612
   json_bit_rate_data = bit_rate.ToJSon(helper.GetOrdering(description))
 
   # Format the messages list with newlines.