Add support for python pageset

tools/telemetry/telemetry/page/page_test_runner.py

Index: tools/telemetry/telemetry/page/page_test_runner.py
diff --git a/tools/telemetry/telemetry/page/page_test_runner.py b/tools/telemetry/telemetry/page/page_test_runner.py
index 33ca9552fe0fe6b25e12c10d4433c18eea6f2a7a..83304e0950afe39d5938b875d984e70b1c79ca8c 100644
--- a/tools/telemetry/telemetry/page/page_test_runner.py
+++ b/tools/telemetry/telemetry/page/page_test_runner.py
@@ -108,9 +108,11 @@ class PageTestRunner(object):
           'pages': [{'url': page_set_arg}]
           }, os.path.dirname(__file__))
 
-    # We've been given a page set JSON. Load it.
+    # We've been given a page set JSON or PYTHON. Load it.
     if page_set_arg.endswith('.json'):
       return page_set.PageSet.FromFile(page_set_arg)
+    elif page_set_arg.endswith('.py'):
+      return page_set.PythonPageSet.FromFile(page_set_arg)
 
     # We've been given a file or directory. Create a page set containing it.
     if os.path.exists(page_set_arg):