Use Welch's t-test to calculate confidence scores in the bisect script.

tools/run-bisect-perf-regression.py

Index: tools/run-bisect-perf-regression.py
diff --git a/tools/run-bisect-perf-regression.py b/tools/run-bisect-perf-regression.py
index b5076bbf3e2dfadb55e022b9f78dc6d7f4f30219..e5621cbb8c7cb0018d31ba373a24b2aa00200aa1 100755
--- a/tools/run-bisect-perf-regression.py
+++ b/tools/run-bisect-perf-regression.py
@@ -91,10 +91,9 @@ class Goma(object):
     subprocess.call([self._abs_path_to_goma_file, 'stop'])
 
 
-def _LoadConfigFile(config_file_path):
-  """Loads the given file as a python module and returns the config dictionary.
-
-  The config file is loaded as a Python module.
+def _LoadConfigFile(path_to_file):
+  """Attempts to load the specified config file as a module
+  and grab the global config dict.
 
   Args:
     config_file_path: Path to the config file.